For some maybe, but most who go far into modding are stick. The DSG
should be able to handle ~320hp and about the same amount of TQs... now I personally wouldn't do it.. I'd get a stick and play it safe..
But then 320+hp in a FWD car is pretty pointless.. you'll have to get an aftermarket diff.. and you'll still be fighting the wheel.. Now an R32 on the other hand..
I think most people stop after they upgrade to a K04 turbine or a supercharger/turbo for VR6s, it's just too much power for the fronts. They then realize they have made a mistake, sell the dub and get an Audi.
I've seen MKI and MKII with VR6 engine swaps, but those guys really know what they're doing. Oh and a guy in Montreal which had a TT drivetrain on his Jetta - AWD and all, pretty sick.
The reason why you don't see many MKV GTis beyond BPU is because most owners don't really care about heavy modding, their car would probably be gone in a few years - no point of modding it and the returning it back or close to stock. On top of that MKVs are still fairly new.
I'm no specialist though.