@MH are you glad you changed from the GTD? I am running the dtuk box like you were also winter tyres to assist with grip etc. Not sure if it's worth the move or not
Mine's had its problems compared to the GTD and you only get to use that extra performance 5% of the time, but when you do, it makes me glad I changed. The mpg isn't as shocking as I thought it would be. The R gets spanked for motorway mpg, that low gearing takes its toll. Equally though, that low gearing makes the car relatively frugal on 30/40/60 roads. If I was a motorway miler i'd say stick with the GTD, but for mixed driving it has to be the R.
Imagine your GTD without the DTUK box on, then remember what a difference the DTUK box made. Then imagine twice that difference and not a twitch from the tyres no matter how hard you accelerate either from a standstill or in gear. The R is amazing.
As for mpg, i'm getting a steady 32mpg tank average. In the Summer with the GTD that was around 48mpg and in the Winter more like 42mpg. The other day I borrowed the wife's A1 1.6TDI to get to work and it did about 52mpg (that was a solid 60mpg journey a month ago). TDI mpg really suffers in the Winter in a way you won't see on the R, making the mpg gap smaller than i thought it was going to be.
You have to adopt a different way of driving though. When you want to really put your foot down, you need to keep the revs above 4k. A rapid 50-80mph acceleration means dropping from 6th to 3rd. Even though the R has plenty of torque (relatively speaking, for a petrol engine), I always find myself dropping 2 or 3 cogs for a rapid spurt rather than letting the torque work the engine in a higher gear, even if i'm only wanting to increase the speed by 10mph.