If the RX-8 overheats, can you use the heater on high to suck heat from the engine bay to cool the engine?

you can

but you shouldn't overheat in the first place.

as said yes....I believe some of the SW guys out there that are running the AP someone posted a thread or a post at somepoint with their reading with and without the heater on and there was a noticeably difference (since they could monitor it almost exact with the AP) but I don't see the need to do it on a daily basis....i think it was just a few degress....10 MAX

It should only make a significant difference in a situation where the cooling system is not coping. It gives you a small margin of extra cooling in that case.

It works for any car.

It will give temporary relief as you are opening up the heater tap which lets out Cold Coolant through the system, and it will take some heat out, but only marginal.

Yeah, I don't plan on overheating, but I live in Phoenix ... so it's always good to know in advance. I had read that that trick doesn't work very well for newer cars like it does old cars; I think it's the temperature controlled cars that don't work as God intended.

If I were in a perenially scorching climate, I'd look into the modifications by MazdaManiac, who iirc, installed an auxiliary radiator. It's the only mod I've seen than seemed to make a serious improvement in the car's ability to shed more heat when the heat load is very high. Using the heater core as an auxiliary radiator isn't a good long-term plan :)

Actually the radiator is from Charles R Hill

I'd rather roast in hell for all eternity than turn on the heater on a hot summer day to keep a rotary engine cool :uhh: