How wide can you go with the stock RX8 rims?
Thinking that 245/40 ZR18 rubber would probably fit the rims, but would this skew my handling?
(It would give almost 2 extra inches of rubber on the road)

 

245 max....handling will feel diff , dont know if they actually "slow" you down on a road course

 

Great for HDPE, however I think the guys might like the narrower tire of 225's better for Auto X

 

Quite the opposite, actually. In Stock class autocrossing where competitors are limited to a stock width wheel, 245s are the norm, and several people run 285s.

That said, 245s fit just fine on an 8" wide wheel, but I wouldn't recommend anything wider for street tires.

 

245 is as wide as you want to go. Toyo and Kumho make great 245 tires

 

I've heard of some guys getting 285 r comps on the stock rims

 

285 will not fit on stock wheel. I've seen guys putting them on wider Enkei wheels though...

 

Iv'e seen 275 with differant wheels too