Is it a bad idea to upgrade the turbo on a stock WRX without upgrading anything else? Its not my car but my friend wants to because the stock turbo looks very small and he can get ahold of an IHI T3/T4 for cheap, im not sure which though. Ill try and get trim and specs of the turbo and post back later. Any help needed, thanks.

he will need EM, injectors, fuel pump, and then if he hasnt already a TBE and up pipe

eM?? electronic management? TBE??

engine management, such as UTEC, or Cobb accessport, greddy e-manage is fine....hydra, aem EMS etc etc something to tune your AF and timing..

TBE = turbo back exhaust, so catback, mid pipe, down pipe, the whole piping

You need loads of supporting mods to upgrade the WRX turbo. Most has been mentioned by Skyline

What a lot of people miss is that the ECU needs work to unlock the power of the WRX. It's like a MAJOR thing. You HAVE to work on the ECU if you want power gains.

exactly....most people think you're just "adding" power with an aftermarket piggyback ecu, but you're FREEING potential stored power by tweaking boost/fuel/timing which is done by either pre-made maps or your own tweaked maps ( potential dangers though)