Why use twin turbo? Seems like a massive ball ache.
I would just get the thing to rev and then have a fairly big turbo on it. Fit a 16v cylinder head and get a bit of boost, should see you at 300bhp no bother I would have thought. Its been a while since I looked but IIRC there are plenty of people over 300bhp with A series minis with 16v heads on and turbos, but they are limited more by their bore size and stroke.
Somthing about the VW scene just seems to lack the performance developments that you see in a lot of other scenes, possibly because its a younger spread of people, or its more about cosmetics. Maybe its because VW didnt really make any cult performance cars like cossies, R5 turbo's etc. I suppose its too easy to upgrade to a 20v turbo.