it's kind of a catch 22 because if you go for the looks of 18 you will be giving up some in the way as performance and traction. pretty much the same holds true for a 17x11 rim as they are heavy along with a heavier 315 tire. heavier rotating mass equals less power actually making it to the ground(not a huge difference but it's there). i have a 315 and they are mainly for show unless you get a DR, traction is no better than my 275/40/17 on street tires. Also lowering your car will cost you some traction as well as your weight will not shift to the rear quite as much. gotta pick if you want it to look really good or hook really good.

 

Exactly, bigger is almost always heavier unless you spend the big money for light weight wheels. You either want the looks or the traction, which you don't get with the low profile tires. If the 18'' wheels are wide enough to get the traction you need with "whatever" tire, you would be better off with a 15" light weight wheel with more sidewall. For every 1lb. off the rotating mass is like taking 4-5 lbs. off the weight of the car. Or so I have been told, take it for what it's worth.