The TT-RS isn't selling in great numbers. A lot of people are saying £42,000 for an Audi that is fundamentally a Golf in fancy dress is way off the mark. So, VW need to put this engine about a bit. It'll almost certainly go into a next generation Audi RS3, but that is still 2 years away, at least. So, it isn't beyond possible for the 5-cylinder TFSI to go into the Golf.
And, yes, a brand new 2.0 litre diesel is on its way for the GTD. My guess is a launch around September /October this year. It'll have the new 7-speed PQ500 gearbox, which will also go into R and GTI models. The new diesel kicks out 2010 bhp. Not quite as quick from 0-60 as the GTI, but kills it on fuel consumption.