Completely agree that the R is good at being slow - pottering around 30/40/50mph roads, it is capable of easily giving you 35mpg, but unlike the GTD, there's a very noticeable fuel penalty from 70mph to 80mph.
I did my sliproad/sign pass speed test in the R today, using a heavy right foot, but not taking it past 4000 revs before the upshift. Without wringing its neck on the revs, the R is no quicker than a GTD going flat out (both hit around 81/82mph when passing the sign. Really wring the R's neck it manages 98mph at the same point - that's one hell of a difference. It's that pick-up above 4000 revs that sets it apart. I suppose that's why the R has disappointed on my commute - there isn't much opportunity to wring its neck on the route.
I did have some fun in the R today, and it showed - 26mpg for a 40 mile round trip, vs 33mpg on my commute. It is good to scratch that itch and get the R, when you consider that to get the same power and more torque, you'll be looking at a 3.0 diesel lump, which would fall smack in the middle of the mpg differences between the R and the GTD - so you don't get that extra power for free, even in a diesel.