My guess is the most grip will be attained by a 17x7.5 and a 225/45R17. You might be able to squeeze a 245 on with an 8" wheel, but you'd have to watch the offset carefully, and probably roll the fender. Tire width is the real issue, getting enough wheel to put the tire on isn't.

-Philip Maynard '95 Contour [71 STS | Track Whore] '97 Miata [71 ES | Boulevard Pimp] 2006 autocross results