the other thing you can check is see if the wastegate actuator is still attached to the wastegate arm thingy.
My g/f's 32 had a similar problem... boosted, then nothing, no more boost, no more whoosh, no power, and a louder exhaust.
At first I thought it was the exhaust wheel had disintegrated. But Burko on these forums had a look, and was in the process of changing her turbo over to my old turbo when, after he removed the turbo heat shield, he noticed the actuator arm wasn't attached anymore. The circlip that holds the arm in place had fallen off, and the gate was stuck open.
Maybe check that?