Well now thats the difference we are talking about. The mk5 had the fsi engine and the mk6 has the tsi.
If it were in anyway possible for VW to get better mpg out of the mk6 gti then im sure they would have done it. 95 or 97ron wont make any difference if its set to run on 95 
OK, faint but pursuing here.
Three potential advantages to the higher octane have been discussed -
- better fuel economy
- better power/responsiveness
- better for engine health/cleanliness
So we've knocked the "better fuel economy" bit on the head as far as the Mk VI is concerned. Personally, I think this thing goes like a bat out of hell anyway (based on a short test drive), and I'm not sure I'd be especially interested in giving it a fraction more. (Back in 1998 my Peugeot wiped the floor with the Golf Mk IV, but it's quite obvious to me that things have moved on and now the Mk VI is considerably nippier than the 11-year-old Peugeot I'm used to.)
So, is the higher octane simply better for your engine? Like its five portions of fruit and veg a day instead of deep-fried pizza? Given that the Peugeot has reached its current venerable age on an exclusive diet of "whatever is cheapest", with no engine trouble whatsoever, I'm sceptical. But I'm interested.
And since I don't actually know what "TSI" is, maybe someone could enlighten me?
Rolfe.