Read the stickies if you haven't already. Also, get rid of the HKS BOV, it is only doing harm to your car.
The basic rule of thumb for getting 320whp from your WRX is:
injectors, fuel pump, turbo upgrade (Blouch makes good setups, particularly the 18G), tune, and intercooler (TMIC or FMIC). But you can't just add a bunch of power without supporting it with other mods. When you get to a power level that high, stock components start to fail. And you can't go faster without touching on other aspects of the car: i.e brakes and handling.
In other words: