I made that jump from GTD to R last May and haven't regretted it. How you do your miles is important.
If you drive mixed roads doing up to 10 miles per journey, expect 28mpg manual and 26mpg DSG.
If you drive mixed roads doing up to 20 miles per journey, expect 32mpg manual and 28mpg DSG.
The second scenario is mile, I see about 32mpg and don't spare the horses, in the GTD, I averaged 46mpg.
As you do 20k miles PA, I suspect you do longer journeys on the motorway and that's where the difference will really open up. I used to get 58mpg on a long motorway journey in the GTD maintaining 80 mph (I suspect religiously sticking to 70mph would yield around 63mpg).
I haven't been on a hugely long journey in the R yet, but the penalty for motorway cruising is a lot higher in the R. Maintaining 80mph would unlikely see more than 32mpg, it is surprisingly economical between 50 and 60mph and then drops right off.