Welcome to DSP!!!
Now get some race tires and leave the STS tires behind! You get to do quite a bit in SP so take advantage of it. You already have the coilovers and IIRC swaybars. You're going to need race tires to be competitive and truthfully, you'll like them better. You can run a stand alone ECU if you want, and technically, you could remove the cats (depending on your emmisions laws). Headers are legal. If you don't have a harness, you might consider one (or two). Brakes are up to you so long as they go on stock mounting holes, so there's room for improvement there. Weight will be your biggest enemy here, since I think you're weighing in heavier than older models? You can remove all sound deadening so long as you leave the carpet (which can be lightweight). Wheels are restricted to 8" wide (i think) and fenders may be rolled but not flared, or something similar. For the motor, you can technically overbore by 1mm, and extrudehone the intake. Porting cannot alter the size of openings, only clean up ridges. short shifters are allowed.
This is all as I understand it. I may be mistaken in some of my interpretations, so it's not the gospel....YMMV.