Tam - have you considered an STi?
Looking in autotrader, the asking price for a low mileage 08-plate GB270 is similar to that of a low mileage 57-plate STi.
And considering your performance requirements, Andy Forrest's STi Level 2 will get you into 340-360bhp territory, without you having to buy a new turbo. To get into the same territory with a WRX/GB270, Andy specs a new turbo (£££). Not to mention with the STi you get more powerful brakes, stronger gearbox, stronger engine internals, DCCD, etc, etc...
Don't get me wrong, the GB270s are beautiful motors, and I can see why they made a fantastic buy when purchased new - but S/H I think you'll get more for your money buying/tuning an STi.
JMHO, FWIW.