The Cobb would work just fine for you power goals.
The EJ20 (stock WRX block) can handle that kinda power but not over a long period of time. And it will take a lot more to reach that goal to than if you did a EJ25 swap (Stock STI block)..your weak point will be your tranny, stock Valve train, and the stock WRX Internals.
PPG's will run ~ $6105 for Dog Gears Helical ($5885 Straight cut) and $4675 w/synchro same for straight.
To reach 375whp you would need to look into a 20G turbo or even a GT35 type sized turbo.
Starting mods to get you in the right track would be Turbo Inlet, Full Turbo Back Exhaust, Headers, engine Management, and an up-pipe.