Hi there! Obviously there are variables but assuming you get a reasonably steady run of it, you should comfortably get 40mpg from your GTI. I've had 50 before, incredibly boring admittedly, and slow, and that was on a 19 mile run on the motorway. In the summer I can get 35 on a 5.3 mile commute from cold! So rather than change and probably lose a big pile of cash, why not see what improvements you can do with what you've got? Mine is a manual, if yours is a DSG then perhaps it won't have quite the same potential but I really do think there's big room for improvement. If you are enjoying driving a bit too much for economy, save that for your days off?