It is difficult to properly cool two spaces with vastly different heat loads and only one thermostat.
Assuming typical computers and monitors and two users, you probably have about a kilowatt of heating or 3400 BTUs per hour.
Rather than run ducts, have you considered a dedicated cooling unit for your office? You can go with a small window unit. Even the smallest one is 5000 BTU and only uses about 500 watts when running. That's just a few pennies per hour and even that is offset by the reduction in load on the main unit. With the thermostat in the same room, its load response will be much better and the temperature extremes you have been suffering with will be eliminated. Plus you can turn it off when no one is there and save energy that way.
Lately my thinking is that ducts are basically parasites and efficiency goes up when the heater/cooler is in the same room as the load. In the past, the equipment cost was lower with a shared unit but operating costs (energy) have gone up a lot (and will continue to go up) and that could change the equation on selecting shared or dedicated equipment. Comfort will always be superior with dedicated equipment.
Given that this question is 3 years old you probably moved on... But I can make a couple of non-expert suggestions...
The first thing I would look at is shade. Can you add some foil into the windows to cut down the heat in the morning? Or some shade cloth on the outside? I have large sliding doors facing west and in the summer I span some 50% black shade cloth from the eave to the ground about 6' out. One can see through quite fine and it cuts down the heat load tremendously (even with the low-e windows I have). This saves a lot of money too...
WRT your specific question, here is what I would do. I you want to go to a ceiling register, check out https://www.hvacquick.com/products/residential/Grilles-Registers/Curved-Blade-Grilles/TRUaire-A303-Series-Triple-Deflection-Curved-Blade-Grilles
Click on the links tab and then on the performance data link. You get a table that shows grille size on the left and then performance data across the row keyed by CFM. Take a 10x10 grille, assume you have 200CFM, that means the air "jet" will reach 12ft horizontally ("throw"), and the noise is "NC 30", which is low (there's no lower entry in that table, but other types of grilles/manufacturers go down to NC 20).
Now the other two interesting measurements are at the top of the table, for that register @200CFM you have a "back pressure" of 0.022 "WC and 600fpm face velocity. You probably have something like https://www.hvacquick.com/products/residential/Grilles-Registers/Grilles/TRUaire-210-Series-Steel-Single-Deflection-Adjustable-Bar-Grilles now, look at the performance info for the 10x6 you have. At 200CFM you have 15' throw (interpolating a bit), 0.026 "WC pressure loss, and 650fpm face velocity. So you would be reducing the pressure by a rather small amount, so you wouldn't get much more air.
Now, if you have an an anemometer to measure wind you could work your way backwards by measuring face velocity. I got a https://www.amazon.com/dp/B01L9SC36A but cheaper will work too. So measure the fpm you have now by holding the anemometer in front of the center of the grille, use the table for the grille you have now to get CFM, then use that to look up the hypothetical new grille to see throw distance and noise level. Note that for ceiling you want curved blades so the air mixes at ceiling level and down't blow straight down on you. You could also go with a round ceiling grille...
If the new grille has lower pressure loss, you will get more CFM, stealing from other outlets. WRT duct, 8" flex doing 200 CFM has a friction loss of 0.024 "WC (http://www.hartandcooley.com/tools/friction-loss-calculator-for-flexible-ducts). Of course I don't know how long your duct is, but it doesn't look like it's going to be a limiting factor, but it may well be in the same range as the grille.
OK, I'm not an expert and an expert would tell you you have to recalculate and rebalance the whole system from scratch if you make a change. And the expert would be right. But you asked how to "wing it, and it doesn't have to be perfect" and I hope that the above gives you a way to make some back-of-the-envelope calculations so you end up in the right ballpark.
Best Answer
Given a circular 8" duct you are dealing with an airflow area of about 50 sq/in. You currently have 72 sq/in opening. I haven't seen a picture of your register - some are not as open as others so it is hard for me to know for sure but... with 72 sq/in opening the air would just dribble out.
This is fine for a floor vent that is unobstructed by walls and furniture but usually not what you are going for. You are usually looking to push the air out a few feet out into the room. A smaller register does this. Also somewhat closing a register does this - however this could push air at unadvantageous angles and you don't want air hitting walls and furniture.
Most registers I size are about the same sq/in's as the duct or a little less. I am looking for the air to be pushed out hard so that it creates airflow in the room. Big rooms with oversized registers can have hot spots away from the register. Now the downside of going to small with your register is that it is too loud. Unless you are severely undersizing your register, the noise is really the only issue.
So a bigger register will not help cool or heat your room better. Putting the register in a better location and making sure the air is shooting out in an efficient manner and at an efficient angle is key.