I'm in the middle of doing a merc m62 20v golf and their is loads of room, the only merc m62 has a clutch pulley which would make life a little bit easier as at a set rpm or boost level you could turn it off, but due to it's large pulley size (110mm) and VW's 140mm crank pulley you wouldn't see much boost. If your target is 600ish then an r32 can handle that easy, I've helped a mate make one with 640hp will post up the plot tomo, thinking back it made over 400 torque @3kish and that uses the biggest T3/T4 no gt bearing turbo.
Going back you maybe better keeping the std ecu and then use another one to control the bypass valve.