2006 WRX putting down 325whp 327 wtq on the 2.5L with catless exhaust, K&N short Ram Intake, Rallispec gears, Stock Turbo, Cobb Accessport Stage 2 and a lot of custom mapping, runs 18psi. I might have missed a few things.
The easiest way to get power is by upping the air to fuel ratio with the cobb stage 2 but this is where the reliablity comes in because of the heat your going to need water/meth injection. my car runs 10.8 WOT and its pretty well balanced for temperature.
Now its true that EVERYTHING ELSE will need upgrade. I added an STI spoiler because the back end does get a bit tempermental at high speed, I have full coilovers because the stock ones are like riding around on mashed potatoes, I just split the calipers and cleaned, resealed, and inspected all the braking components (They get EXTREMELY hot) 600+ Farenheit. And thats with the 06 upgraded brakes which are much better then the original WRX ones (I had an 02).
So is it possible to make 320WHP on a WRX 2.5 motor, yes. on a STI motor, yes. On a 2.0, yes with a intermediate sized turbo. But then you have to worry about tires, wheels, brakes, bearings, suspension all being at their limit. With a lot of work and care you can have one that is both reliable and a power monster it basically comes down to eliminating heat in these cars they produce so much that it just tears components apart.