Cannot talk about the MK 8 GTD but moved from Audi A3 Quattro to the MK 7 GTD (63 plate) and then onto an R and the TCR and can say with certainty that the GTD never ever felt it lacked overall and very much held its own, in that the essence of the what made the MK 7 great was retained completely, the cabin being a great place to be, the ride the same on DCC and despite down on BHP, the 185 BHP the MK 7 had still felt quick, certainly for normal roads, and its on the move pull with the diesel torque was again quite decent.
You basically just missing the outright pace of the GTI and R and slightly lighter handling of the GTI, when really pushing on, but little else, and when just moving around like everyone else, the driving experience differences are quite minimal, you in a nice cabin, with a great ride.
The bonus as doing more miles at time was the 55+ MPG that got on the medium to long runs, though that dipped to circa 38 MPG on local runs (context is that get about 28 MPG on local runs in present GTI)
GTD is probably the best all around car in the performance Golfs, and if was given one would be more than happy to receive.