The GXP is already loaded with most of the goodies, drilled rotors, bilstein struts, good springs and a lightweight set of alcoa wheels.

You can get a K&N or other intake, maybe a hi-flow cat (Magnaflow makes good one...possibly even one made into a bolt-in downpipe)

Magnaflow and others also make good hi-flow mufflers but the Summit Fully Welded muffler is a hi-flow bargain.

i'm not sure if anyone offers a PCM for the car or maybe you can get a capable tech to re-tune your present PCM. Just keep in mind that the FWD trans is your weak link. 323 ft/lbs stock is already taxing the limits of the trans so be a little ginger on the hot starts.