Today I commuted to work on Eco mode in the R (more to get a feel for how the car will sound with the Soundaktor off) and it did 29mpg without really altering my driving style. We were a little later in setting off, so the free-flowing traffic was moving a little slower (maintaining 65-70 on the coast road). This compares with 29mpg achieved in Race mode yesterday and maintaining a quicker cruise for the 5 miles on the coast road. Eco mode seems utterly pointless, for the shorter journeys at least.
Not surprised really, it had no benefit for me on the GTD either – but it really blunts the throttle response.
Last night I had a leisurely drive up to Blyth to pick up a new rabbit cage (we got another house rabbit recently), driving along the North Sea coast via Tynemouth, Whitley Bay, Seaton Sluice and finally Blyth (11 miles each way). There was no opportunity to put your foot down apart from a small stretch of dual carriageway near Blyth (so I did), pootling at 30-40mph most of the way and got 35mpg. Obviously nowhere near the GTD, but demonstrates nicely that (for a high powered petrol car) it can give you some decent mpg returns if you’re not pressing on (and you don't need to be in eco mode to get it).
The R seems to get to around 22mpg very quickly (within 2 miles), and then you see the small incremental gains as the journey goes on thereafter.