Links below for what I've done.
I wanted a quicker car relatively inexpensively without having to make significant other mods like replacing the clutch. Done with the Phantom ESC and Open Flash Tablet for $2500.
I wanted more bass response. Done with a 10" powered sub in the trunk for $275 installed (and it wedges perfectly under the back deck).
I wanted less rattles and lower noise inside. Partially done with sound dampening insulation in the doors, trunk and under the rear seats. Still intend to do the front floorboards (total cost about $75). Also need to figure out how to resolve the occasional rattle coming from the back deck (I know there's info out there about it - just haven't gotten to it yet).
Otherwise, the only things I think I'll do is put on an Open Flash Header and Michelin Pilot Sport AS/3's at some point. Love the car.
Phantom Electric Supercharger:
http://www.phantomsuperchargers.com/...ubaru-brz.html
http://www.ft86club.com/forums/showthread.php?t=39719
http://www.ft86club.com/forums/showthread.php?t=66862
Open Flash Tablet:
http://openflashtablet.com/Automotive/FRS/index.html
Amplified Sub:
http://www.onlinecarstereo.com/CarAu...e_P300-10.aspx
Sound dampening:
http://www.jcwhitney.com/bonded-logi...&filterid=u0j1