The 18x8 combo seems to be at the limit for the TT Quattro with stock offset. I was at AUDI AG a month ago and spoke to the engineers about offset for the TT Quattro.

They commented that I would be better off with the 18x7.5 and install H&R spacers (Front 20mm / Back 25mm)

This has brought the tires even with the fender without changing the handling. The car has also been lowered using H&R springs. The car rides incredible and looks even better.