How much money are you prepared to spend? The more invested = more power.
Where would I start? By reinforcing the engine. The ej225 is notoriously weak; chocolate pistons is the phrase I hear a lot. Once you start running more power through it, it will pop.
That means replacing internal parts with forged parts. Pistons, rings, rods... etc.
Once you are properly strengthened you can start to up your power.
Most simple is a remap. Smooth your power band and up the boost pressure. This will up your power a bit and probably won't need internals replacing. However if you plan on doing other mods the remap needs to be done last.
Larger exhaust. A 3inch bore, from cat back to stay legal, will help release turbo goodness. A sports cat will also help. These help exhaust gasses escape and this is good.
Upgraded fuel pump to deliver more fuel.
Upgraded fuel injectors to get more fuel into the chambers.
Larger turbo to deliver more boost.
iirc the US WRX doesn't have a twin scroll turbo so you may want to consider changing to one of those instead - less lag and more boost.
All that will probably see you into 400 bhp; this, however, is the area of the ej225 going bang, so be careful if you've not forged your engine by this point.