The base GT come with poor 235/50/18 tires and narrow rims and IMO the limiting factor to all performance of the car in stock form. Rims and wide sticky tires as your first mod will be a large and very noticeable improvement to all performance aspects of the car, cornering, accelerating, and braking. Do your research on sizes that will fit and budget ~3000$+ for it.
All the peeps talking about suspension first are spot on. Add power and have no control? No thanks. As [MENTION=31830]Austinj427[/MENTION] says...CB005...lock down that when the bough breaks, the CRADLE will fall...lol. Now don't get me wrong, power adders are definitely worth while, but IMHO you need to be able to control that power. Look at my mods...no power adders yet and from what I and many others feel, are "must have" mods; I have 6,500 miles on it and I feel like I'm ready for the power adders. I'm really glad I got to know how my ride deals with the power when I stomp the go fast pedal since it does comes stock with a lot more power than most vehicles on the road today.