These are some of the things I did on my rev D (2001), your's looks like its an early rev C or late rev B
Hose 10 mod
rear sway bar
redo vac lines
re plumb BBOD
relocate boost solenoid
lower it (see if you can find some sti springs)
sti bucket seats
genome muffler (I also had some z/s dps, didn't make much difference)
re do silicon elbows under intercooler
clear side repeaters
fiddle with the ECV rod