Be careful slinging around that epitaph 'a lot of wrong answers'. Intake fans do not do much at all for what most ppl would consider 'air pressure. Air pressure itself is 14.7lbs/in². What you are calling' positive pressure' is closer to 14.7001 lbs/in² and 'negative pressure' would be 14.6999 lbs/in². That's about enough difference to affect the microparticulate dust and thats about all. There's far too many vents, gaps, fan slots, grills etc for there to be any consistency in airflow or for pressure to have any affect.
Take a good look at a case, any case with side fan vents. None have dust filters. Even cases claiming to have full dust filtration do not have filters on side vents. They are meant to be used as exhaust, not intake. The only time those side vents were used as intake was on old AT cases and first gen ATX before case designs really took cooling cpus a little more seriously. Cases are all about cooling the cpu, because that's general to every pc, large, hot gpus are not present in every case.
Using side fans as intakes keeps warmer air inside the case, putting more emphasis on the cpu heatsink. As you say, heat rises, you might have cooler running gpus, but the cpu suffers as a result. Flip those fans around and the top gpu will see less of an impact from the exhaust of the bottom gpu, and cpu temps at load will be lower.
Heat rises, that's a given, but if you want to see the effect airflow has on heat, light a cigarette, hold it upright and look at the smoke trail. Goes up pretty solid, all by itself. Now blow across the top of the cherry, the smoke goes everywhere. This is what happens to natural thermal conduction with the introduction of a side fan. You blow the gpu exhaust heat throughout the case. Some goes out, most doesn't make it that far and gets recirculated back to the gpu/cpu intakes. Now try sucking air across the top of the cherry. Smoke pretty much goes with the airflow, most will make a 90° turn. That's what happens when the fan is used as exhaust.
Better by far to use side vents as exhaust unless gpu temps are so out of control that they require the fans used as intakes.
Nature abhors a vacuum, air will always flow naturally from higher to lower pressure, there's no force involved.
This from an expert btw.
Edit: oh and BTW, gpu fans face down, but blow air upwards into the gpu, not down to the case bottom, might wanna rethink that statement.