If its running 270-280bhp then I'm guessing its at least got a sportscat (or decat) and a remap? (as they're about 220bhp standard, 250bhp if fitted with PPP).
Power-wise, you're pretty much at the limit of the TD04 so to push over 300 you'll need to change the turbo. Usual route on a WRX is VF35 with STI pink injectors, decat and further remap to see you about 320-330bhp.
