If you are that bothered about fuel consumption, a GTI isn't for you.
You do have to evaluate such things though imo i used it too work out how much a year extra a petrol would cost me over a deisel.
My figures brim to brim average over the same route over 6 month period, These including a commute into town early but back out in rush hour.
Mk1 1.8 GTI 38mpg average
Mk5 2.0 GTI TFSI manual 33 mpg average.
Mk5 2.0 GTI TFSI DSG 29 mpg average.
Mk5 2.0 TDI 140 manual 43mpg average.
Mk6 2.0 GTI TSI i wish i knew but it is suppose to be a bit better than the mk5.