Any major gains are going to start after you get rid off the factory fuel injection system(restrictive air flow meter) go with a Go-tech (which I have) or a mega squirt, that will allow you to place a nice non restrictive intake.
then the rest depends on your budget. there's plenty of guys here that can help you get practically anything you need for performance upgrades.
not to scare you ,but performance parts for these cars are not the cheapest.
talk to JJ (junglejustice) he'll lead you in the right way when it comes to juicing up your car.