the Gm bcs would be good for the 2.0 to get spooling and hold boost better.
changing the bcs is not going to help boost oscillations if not tuned properly will make boost oscillations worse.
i personally would not run the PW CAI with a off the shelf tune
you can use your friends cable but for the 03s i believe you will need a different jumper block.
the xpt maps are good safe maps but you still may need to play with it a little to get it to work with the car.
__________________
6 letters spell it, 5 lifters be ticking on it, 4 cams spinning on it, 3in cutout on it, 2 heads flowing on it, 1 turbo creeping on it, SUBARU.
Osecuroms.org Opensource Stage II 2006 wrx 281 downloads and counting
Pm me about Subaru Tuning