I went from the stock 17" wheels and 245/45/17 tires on all four corners to the following setup:
18"x8" wheels, 245/40/18 Toyo T1-S in front
18"x9" wheels, 275/35/18 Toyo T1-S in back
And noticed the following differences:
The wheels and tires look awesome. Much more "aggressive" than the stock setup.
The ride is noticeably rougher, but not unpleasant.
The 275's are harder to turn over, but on a "slick" surface will still squeal and smoke.
When inflated to 35psi or higher, the lower profile tires corner fine. The stiffer sidewall helps prevent rolling the tires off the rims. (40psi is recommended for "hard" driving, i.e. SCCA Solo2.) Anything less than 35psi makes me nervous, but may be OK for everyday driving.
I am told that going from 17" to 18" wheels makes the car slower, but because I went to a lower profile tire, the actual wheel/tire diameter DECREASED by 0.1". That should make me FASTER, right? (Assuming the new setup is equal to or less than the stock weight. I'll weigh them this weekend.)