Yeah, tires make a huge difference. You are going to have a very hard time finding anything in a 15", though. You can get low-profile tires in 15", but they are very small, and no good for street use. Your best bet if you want to run stock is to get some extra 15" wheels and 205/50R15 Azenis 615s or Hankook Z212s. They are tiny, but that won't hurt you on the track. They also are stock class legal and cheap. If you want to go STS, get 16x7.5 or 16x7 wheels and 225/45R16 Z212s for a great bargain in traction.
