I did the 2003 GLX 4Motion mod as follows:
EvoMS intake (270.00)
GIAC chip (300.00)
Unorthodox Racing Under Drive Pulleys (400.00)
Remus rear mufflers (4Motion, of course)
Bosch Platinum +4 plugs (not much money on that one - 6.00 each?)
Was going to get Tip locked in Sports Mode, but never got around to it.
People will suggest PES Supercharger, but you are talking 4,000.00 (290bhp).
Good luck, and welcome to PW!
Read a lot, consider what you expect v. what you want to spend.
I wouldn't put the money in to it if I had to do it again.
Especially since I traded it in a month ago on a W8.
But the "mod-bug" lives on: GIAC flashloader with 91-93 octane program, stock program, and 100+ octane race program - and Tip locked in Sports Mode.