I'd suggest like above a VF35 (or 34) turbo to reach around 330BHp - however you need to factor in:
- uprated injectors (STI Pinks)
- uprated fuel pump (Walbro or similar 225)
- Sti TMIC (including STi scoop and undertray) or other improved intercooler solution (e.g. HDI Front mount) -
- Replacement induction hose - you will probably split yours changing the turbo!
- Exhaust (just go for 3' from the start)
- De-Cat Up-Pipe
- 3 port boost solenoid
Optional but just as important imho
- Uprated Clutch
- Uprated Brakes (Brembos, K Sports etc.) which usually means changing from WRX wheels
Don't forget WRX gearboxes are pretty weak (350 is about the recommended limit) plus the internals don't like 370BHP! (experience speaking).
Do the maths and have a good think is it best to tune what you have or sell it and buy STI (better brakes, suspension, gearbox, diffs, internals) - I know what I'd do now if I could travel back in time having got my WRX to 450!
__________________
2005 Spec B 6MT - KiDO tuned (~270Bhp)
Custom exhaust - Cosworth filter - Samco Intake - Tints - Bilstein B8/B6 & STI springs - SuperPro ALK - Whiteline positive shift kit, rack mounts, rear adjustable camber bushes, front ARB mounts & strut brace, 20mm rear ARB with AVO mounts, - Hardrace Gearbox and pitch mounts - Prodrive PFF7's - Front Brembos - Kenwood CarPlay - 1/2 carbon grille - STI front splitter