i like the look of the stock 17s fine. if i go for some nice tires (nitto 555) would i be far behind 18s performance wise? it will save alot of money and as mentioned above the 17" tires are much cheaper.
Performance wise, teh 17 is actually better for straight line accelerating. Reason being that on the 17, you can get a taller sidewall tire on there (such as a drag radial) than you can on an 18. And typically a 17 has less rotational mass than an 18, unless we're talking about a very high dollar lightweight wheel.
What's most important for hooking, though, is the width of the actual contact patch, and the compound of the rubber, IE how sticky it is.