Yes, I have an S1 but there is no realy difference between the turbos, some will say there is because of the plastic compressor wheel in the GTT vs GTST, but who cares, we are talking about the turbine wheels which are both ceramic.
I havnt heard anything in specific, but think of it this way. The turbo was made for the car in stock for, designed for it in stock form. So it was ment to run lets say 7psi daily. Now, you boost that to 12-14psi, thats 80-100% MORE then what it is stock.....in my experiance anything pushed to double "reccomended/designed" has not been as efficient/capable etc.
Also, the same goes for the flow of the intercooler, but a GTT intercooler is good for around 200awkw so word goes.
Its not just boost which brings power, its the flow of a turbo. Thats why people say "it flows 450hp". If the turbo can physically flow more air, you are removing the biggest bottle neck in the system (lets say the pipes are the upper limit). So, with a bigger pipe (assume turbo has reduced the bottle neck) for the same pressure you can flow more air....more air is what causes more power. Thats why GCG bore out the turbo compressor/turbine housings to fit bigger steel wheels. If you just replaced the stock turbo wheels with steel units, you wouldnt get much if any benefits.