Saw something interesting on a Mazda 6 advert for their 150PS 2.2 Diesel today, boasting 67mpg economy(comparable to the GTD, but with 34PS less).
They had a prominent arse-covering disclaimer stating that published mpg figures should be used for comparison between car manufacturers only and may not reflect real driving experiences.
With this in mind and frequent postings for poor economy on MK7 Golf on other forums, modest gains over MK6 seem all anyone here can hope for. The official figures for mpg have more to do with reducing CO2 figures than expectations in normal driving. My dad has been disappointed with the mpg (but not the performance) compared to his MK5 170TDI, can't see it being any different for the GTIers.