The wall space clearance requirement of 30" is from the floor to ceiling. And the panel does NOT have to be centered. Unless you have like a 6' ceiling or a lot of things like counter tops in the way etc, I don't see why this would be a problem.
The process of upgrading from 100A to 200A is not just a swap of your panel. It's a little complex and most definitely has to be permitted. It is doable yourself. In any case, it's good to understand the steps whether you do it yourself or hire an electrician:
Check with the utility company to see if your "drop" (wire form the pole to the house) is capable of 200A. Normally, a utility company will have to upgrade this themselves. But sometimes you'll have to pay.
If you have a separate meter box with main breaker and the separate panel in the house, the line between the two will likely need to be upgraded to 200A line.
You will need a 200A new panel in the house and all new breakers that are up to code.
You will need a new meter box and 200A compliant meter from the power company.
You will likely have to upgraded / bring other things into compliance to pass inspection. Such as, the ground wires. Many areas require you ground to your main water supply coming thru the foundation along with the two grounding rods. You may need to install special breakers if your bathrooms aren't GFI'd for those circuits.
Basically, you'll have to pull the permits. Have the power company pull the meter which will shut off the power. Install all the new equipment with the power off and then get it inspected. In my area of the country you can get what's called an "open circuit permit." Basically, you can hotwire your new 200A meter box so that you can get your power back on while you wait for an inspection and for the power company to install the new meter (they generally won't install it until you've passed inspection). This just means you're running power for a while with no meter and being charged an "average" rate.
I am pretty handy and wouldn't tackle this myself. Though, there are some good youtube walkthrus and it can be done. If you price it out, keep in mind the prices will vary wildly. I had my house done for $3000 and that included eliminated a remote panel in my basement and a few other upgrades aside from the 100A > 200A change. But, i got several other quotes and they were: $4800, $12000 and $6700. Like I said, get lots of quotes! Figure on at least $3000.
Spaces vs Circuits
First let's explain the 8/16 thing, or "8 spaces but 16 circuits". At some point, panel manufacturers developed what I (alone) call a "double-stuff" breaker, which crams 2 breakers into a single space. This is intended to solve crowding problems in too-small panels.
photo source: Home Depot
Relying on these, marketing called their boxes 2X-circuit (X-space). Around 2014 they mysteriously stopped doing that. Recent NEC changes required AFCI or GFCI breakers on damn near every circuit... AFCI and GFCI are not made in double-stuff breakers.
Your 16/8 service panel is a legacy of that bad labeling. You have an 8-space panel, intended to replace an 8-space fuse box.
Panel space is dirt cheap. Go nuts.
You are dreaming of a 16-space panel. And even that is dreadfully small and you will find it rather limiting even with 100A service.
The practice of many electricians is to give you the smallest panel possible. That's first, to force you into more subpanel work if you expand further (more money for them) and second to save themselves $20 in part cost.
The cost of a larger panel is trivial tiny compared to total project cost. It is very correct to be extreme. Please spend an extra $100 for a 42-space panel, unless you just can't make it fit. Honestly I'd go 60-space. Not least, those larger panels are 225A-ready.
You never know. You might do a kitchen remodel and go hey, I'd like a separate circuit for refrigerator done microwave done dishwasher done disposal done 3 outlet circuits done. You have the space.
When you buy a PC, do you really calculate your needed hard drive space and buy just what you need... Or do you just buy the big 3TB drive? Of course you do. Same thing here.
More work to do at the meter
OK so the power company says their side of the cable is 320A. If they say so.
They say their meter is 320A. If they say so.
But your meter housing is not 320A.
To support more than the listed 125A, you will need to upgrade your meter housing. There's no question of that. That's your equipment and you have to pay for it, probably, unless your power company does something different with cost sharing.
You won't be able to replace parts of it, you will probably even have to replace both sides of it as a single unit. However it is very nice to have your main breaker in the meter. It means you can fully de-energize your main panel, which makes it safer for you to do yourself.
The 320A may be shared
Since it's multi-unit, the power company provisioning 320A makes more sense. Trouble is, this 320A service is probably shared between both units. So it's not as much as you think.
You really need to talk to your power company about what they consider this "320A" to be. It may be a relabeling of what other power companies call 400A service. So they may be willing to feed two 200A panels, or 100/100/200. This is a conversation you can have only with them.
More food for thought is sub-metering units, and separately metering common spaces. The new thing in rental properties is to have one main meter and the landlord has sub-meters per tenant. Many landlords prefer (and some law requires) commons space usage to be on a separate (sub) meter, i.e. heating, yard and commons lighting, anti-freeze pipe wrap or roof/gutter heat, coin laundry, Christmas decorations etc.
Best Answer
Short answer is "no"
There are a couple ways of fixing this short of going to 400A busbars in the main panel, which are not exactly the cheapest thing in the world. Read on for options, although you'll need to discuss both with your AHJ and the latter option (using the secondary disconnect on a suitably equipped meter-main for the EVs) with your utility as well.
One other caveat with most of this is you're best off with 32A EV chargers (vs 40A) to avoid starving the house for power too much -- luckily, 32A@240VAC is a standard circuit size in most of the rest of the world, so you can get EV chargers that size fairly readily.
Easy fix -- downgrade the 200A feeder breaker in the main panel to 125A
Given the rules of NEC 705.12(D)(2)(3)(c) and that the total overcurrent protection for the planned EV chargers won't exceed 100A, the easiest solution to this problem is to downgrade the main panel's feeder breaker to 125A, which brings you back into compliance with the rules. This does mean that if both the Powerwalls are charging at their full 30A rate, your solar system isn't helping at all, and you're pulling significant load (more than 65A) on the house side, you could trip the feeder breaker.
If you are OK with starving the EV chargers for current somewhat, or your AHJ is willing to apply 240.4(B) rules to this situation, then you can use a 150A feeder breaker, but that limits you to 70A (without 240.4(B)) or 80A (with 240.4(B)) maximum breakering for the EV chargers, taken together. (If you could put a 75A breaker in, that'd work, but 75A is not a standard breaker size.)
If that's not an acceptable state of affairs...
If the above situation is not serviceable, then there is another option, but it involves replacing your meter socket with a specific meter main (or meter breaker), which may or may not be more costly than the 400A panel. In particular, the Eaton/Cooper 2M2RP provides a 200A continuous socket with two load taps: one for a 200A maximum main breaker, and a second provision for an auxiliary service disconnect. However, the lugs on the meter socket are limited to 125% of 200A (their continuous rating), or a 250A maximum, so the disconnects must sum to that (vs. the 225A maximum rating on your proposed main panelboard's busbars).
This means that you can use a maximum of a 175A main breaker in the unit (downbreakering it from the shipped 200A), and the auxiliary disconnect paired with that can be a maximum of 75A as a result; however, 75A isn't a standard breaker rating, so we can use an 80A breaker instead as per 240.4(B) (it's a feeder, we're protecting a set of lugs, and we're below the 800A maximum for the rule). Given that the 2M2RP takes Siemens breakers (for historical reasons), and that Siemens has replaced the QJ line with the backwards-compatible QR, we need a QR22B175 for the main, and the provision for the secondary main can be left unpopulated for now. When the EV chargers are installed in this scenario, a Q280 is inserted in that secondary main slot, and then a feeder is run from it to a main lug panel with the EV-charger branch breakers in it.
If your AHJ or utility rejects the application of 240.4(B) here, one can use a QR22B150 for the main and a Q2100 for the secondary, instead -- this does mean that pulling full power from the house while the Powerwalls are charging at full rate and the solar system is outputting nothing puts you in main-breaker-trip land, but that is quite unlikely. Another option would be to use a QR22B175 for the main and a Q270 for the secondary, but that starves the EV chargers for current ever-so-slightly.