What mods are you looking for? Many of the suspension mods will work on
your car. You might check for accessories at www.subarugenuineparts.com 
. Also, the normally aspirated (non-turbo) forum at www.nasioc.com may
have some ideas. It's unlikely you can get more than a few HP with
intake/muffler mods. Removing weight  (especially rotating mass) will
help. you get about 1 HP for every 8 lbs saved (+/- 2-3lbs,
rule-of-thimb, not a linear ratio) Do you really NEED that back seat?
Passenger seat?  ;^)   . Maybe lightened crank pulley and lightened
flywheel will get you a little faster acceleration.....maybe, lighter
rims can help PLUS you get a more custom look.

Carl