Wheels!!! Lighter, Wider wheels. 17's or 18's both give you a much better choice of rubber to put on the road (a 17" rear will give you the best choice since almost every real sportbike made runs a 17" rear). Either way it will improve your bike in every catagory. Less rotating mass is good!! The bike will accelerate better, handle better, and stop better.
Not the cheapest mod but IMHO the single best bang for the buck mod you can do.
What's the general price range for this? I'm assuming tires will be $100-200 each?
Besides new wheels what else would need to be done/bought? You can't just buy different size wheels and stick them on, right?