I have 205/50-15 Hoosiers and 205/45-16 Kumho's that I use on my RX7, both on 7" wide rims. I might get away with a 225 in the rear and be faster, but the 225 will lose crispness on the front of the car.

With that said, I also used to use 225/50-15's on 15x7 rims on my Mustang GT when I used to run it is stock form. I ran against lots of people using 245/45-15's at the time, and I will tell you that I never lost to them in an autocross. I felt with the 225's my turn in was much crisper than theirs (and I think I proved my point). The Mustang is a 3250lb car where the RX7 is in the 2800lb range.