We have done 245/40 on 17x8, but it feels sloppy. That 245/40 would be faster for autocross than 235/40, it's just the nature of autocross as you know that we stuff extra tire on there to go faster despite fact it feels sloppy. Website wheel section HERE
says max fit under stock fender lips is 235/40 and we are sticking to that position. So, if you don't want to worry about the potential rub my suggestion is the 235/40 size (Dunlop makes a good 200 treadwear, NITTO makes a good track tire in that size, Toyo has a full race RR in that size). If you want to run the Rivals/RE71R in 245/40 my suggestion is roll those fender lips to avoid tire yanking down the lip and breaking the paint.