[tguvot]

>Our walls are good but our windows are a weak point.

are you sure ? at summer I have walls that are 170F outside (at 2pm) and 80-90F inside. Also, window will pass through heat only during the day. Wall will keep heating your inside air through the night as well. Get some cheap thermal camera - it's very educational.

You should also check your ductwork/registers. To see if there are any tears/openings/etc. Also, in case you are using flex duct, if it's not tensioned properly, sagging, etc - there will be a lot of air velocity losses that reduce overall cooling experience.

>Any idea why the HVAC guy and just about every source online insists that A/C can only give you a temperature delta of 20F? I know it can’t be true, but that claim is everywhere.

Something around refrigerant properties, mechanics and physics, etc. I also spent some time in this rabbit hole