I just went from '06 WRX to an '06 350z to an '07 GTI. After messing around with the GTI's DSG launch control, I would have to say a stock WRX would probably be pretty even with the GTI.
A good launch from the WRX would probably cement a victory, but from my experience with the WRX, I had a hard time getting consitant launches.
The GTI DSG is super consistant. That said, an 06-07 WRX would probably beat an '07+ GTI in s straight line, but it will, by no means - "hand a GTI its ass."