Given the amount of soot and **** that sits around the tail pipes of my exhaust, whilst I accept some of it will be unburned fuel from short journeys, I hope that some of it will be whatever the V-Power cleaning detergents are cleaning out of the engine. These engines seen quite bad for carboning up due to the unique way Audi seems to have designed the FSi system, so based on that I try and use one tank of V-Power to every 3-4 tanks of Momentum 99 which is 4p a litre cheaper on average round here.
I've never believed that anyone would be able to tell the difference between 97/98/99 RON, but if you run these high octane fuels and then go back to 95RON car you can tell a difference in the urgency. I ran Shell 100RON in Germany this year and the pull at the top end was noticeably better on the autobahns (much stronger pull higher up the speedo).
If you read inside the fuel flap of the 2.0T's I'm sure it recommends 98RON as the preferred, and then 95RON as a suitable alternative. Pick one and stick with it to save the ECU having to go through continuous adaptation. I feel sorry for Americans, their fuel may be cheap, but 91RON must feel pretty weak in a performance car.