The SVT is a great car, but the downfall is that its really sort of "maxed out" stock from the factory. It breathes well, and it handles well. Most of the mods that will make the most difference are things like a short throw shifter, better audio equipment and headlights.
Most of these mods are just comfort mods that don't really affect the performance of the car, they just make it a better place to be.
Hands down the best performance mod you can do is suspension. The stock setup is good, but a proper suspension setup will turn your SVT into a corner-monster. But besides that, the only other thing I recommend is proper maintenance.