Sounds like the diverter valve may be gone or the turbo knacked? does it lack that typical turbo boost when you can hear it kicking in?? any black smoke?
The turbos aren't generally that noisy, so something may be amiss.
On the Eddys, it is very easy to check the diverter valve, it is at the front of the engine just behind the rad on the RHS, 3 bolts and it's out!!
It may well be the diaphragm torn, and this is easy to see once taken out.
Google it, many photos of torn/damaged ones, and how simple/cheap it is to replace.
Slightly more difficult on a standard Gti, as it is under the car!!