There just about ok at standard power. In all honesty after I suffered brake fade and had Brembos sat on the shelf, next step was to ditch them.
Brembos are far superior to standard calipers.
Instant stopping power when equipped with decent pads/discs.
I personally wouldn't suggest someone wasting money on trying to enhance standard stoppers. Ditch them and buy LCR Brembos as a minimum.
As above brake technology has moved on. I had the same 312mm ATE brake setup on my PD130 Passat it was standard fitment on that car. A Passat carries a bit more weight than a TT but not that much really. I have swapped to the LCR setup on my TT which I intend to track and will hopefully be remapped shortly. The LCR setup feels a lot better than the standard brakes and Seat thought that it was the brake setup that fitted the performance of the car .