A GTD R is wishful thinking.
Yes, the engine may fit the Golf. But the issue is the corresponding gearbox.
With 500 Nm of torque it would need the same DQ500 DSG7 as in the Passat
and 4WD. I doubt this DSG gearbox will fit the smaller Golf easily.
Even more, you'd have to pay 5000 € more than for a current GTD DSG at least.
See the price difference on the Passat engines (190 PS DSG FWD vs 240 PS DSG
4WD).
Who's willing to pay that amount of money on a Diesel Golf? Yes, there are few
addicts who would. Because they love Diesels. But the majority wouldn't pay
40.000 € for a car which isn't as quick as a Golf R. With 240 PS and at least
1500 kg weight even a GTI would be quicker.
It's not the point that I wouldn't spend money on such a car. It doesn't make
sense for Volkswagen.
I'd say, have (more) fun with a Golf R or be sensible and safe fuel with a GTD
or GTE. If you can't decide between, get a GTI. Or so.