I don't know why they didn't use Pirellis like the WRC car does, but they are the best, IMO.
Trunkmonkeys way is safe. But I like power, (and a rear swaybar makes you oversteer more, not always a desireable trait.) So I would upgrade the suspension and power in equal amounts, with some brake work. Here is my list:
tires
some revised suspension (built to what you want, like coilovers from our friends at Koni)
tougher brake pads and rotors
air intake
exhaust
chip (basically, pump up the boost!)