suspension mods for handling, springs, adjustable shocks/struts. Get a fuel and ignition management computer that you can get tuned for your turbo (c'mon, the turbo's a given). Injectors, many if not all of the previously mentioned items (cold air intake, turbo, etc). Port and polish the heads/manifolds. etc. etc. etc. Many things you can do, I think that's why they call it customizing! Have fun!