HVAC system setup

First, realize that with a single zone system and multiple floors, it's difficult to get the temperature correct everywhere. Also, since you mentioned this was an attic space, you're likely against the roof and getting heat from every wall plus from the computers and people in the space. In short, you're fighting a losing battle.

I've got a similar challenge, though not to your extreme, and here's what I have done. First, learn how to adjust the baffles on the ducts. In the summer, you want as much as possible going to the high floors, and the reverse in the winter. Get someone to feel the air coming out while you're pushing the baffle all the way to one side or another. For me, it made little difference until I pushed it all the way.

Next, exclude any rooms you don't need to heat/cool, e.g. a guest room or storage space in the basement. Shut the door and close the vents to reduce the effort you're putting on the HVAC.

Finally, I swapped out my return grille with models that accept a filter (they are hinged and open after releasing two small clips). These return grilles are special orders from the big box stores, but they carry them. I still leave a thin blue "rock catcher" filter directly on my HVAC blower itself. And I put a filter in every return except the upstairs in the summer and downstairs in the winter. The idea being to suck out as much air as possible from location that needs it.

This means that switching between heat and cool is a process, flipping the baffles and swapping in and out filters. But a little effort is better than a lot of sweating.

One last suggestion, since it sounds like you're running a small server farm in your attic. Either move that down to the basement, or consider replacing the window unit with a ductless mini-split system. You get to see out the window, and these systems have configurable thermostats.

Does the ratio of static pressures between two stories differ at different blower speeds? That's a very interesting question, but I believe it's a moot point. Any dramatic difference would mean your system isn't balanced correctly in the first place. (needs zoning, split systems, or an actual balancing)

Stack effect will cause plenty of heat to go upstairs, while the cold air will settle. I'd be more concerned that it won't cool right. Except that furnaces usually only run at high speed for AC because cold air is denser, making it harder to push. If your old system heats and cools just fine I don't foresee a problem.

Without being in the correct centralized location (which is somewhere on the first floor of a two story), the thermostat will not control the house properly. In low-fire mode, indeed less air will go upstairs and downstairs, per minute. Blowing softly is what makes it more comfortable. It will run longer and more often, reducing the intermittent temperature differential, keeping the house nice and even.

So no, the same amount of 'less' air will be coming out below as above. Your contractor either is trying to up-sell you into a split system (or knows more than I do) or convince you that zoning is required (a possibility, however this can be retrofitted later at your discretion). Either of these additions would dramatically increase the cost.

Unless your house has automatic zoning and was balanced by a genius, it will always be hotter on the second floor in the winter, and colder on the first in the summer, just like everyone else's.