People seem to get all bent out of shape over the choice of what is effectively the same thing, even the difference between 95 and 99 RON is not as dramatic as the hype would lead you to believe although I accept for modified cars where the map has been written specifically for 99 RON the difference will be more apparent.
The 2.0 TFSI engine, found in the GTI's, S3's, repmobile A3's and A4's will handle 95 RON without any hint of problems whatsover. In fact the engine is designed to adapt itself to whatever its fed.
Furthermore based on my real world click to click mpg figures there is feck all difference between the fuel economy you get with 95 RON or 99 RON. The difference is so small it absolutely doesn't outweight the additional cost of 99 RON and indeed leaving your car idleing for a couple of minutes more, the temperature outside, whether you have a boot full of crap, whether you have a passenger or not will completely obliterate the tiny improvement you might make.
From a performance point of view mine feels a touch more lively at the top end of the rev range on 99 than it does on 95 but as most of my driving is a fairly sensible commute its not often I stray into that range.
I believed the hype on 99 and fed my car that religiously for 3 years. Then I was forced to use 95 for an extended period of time and suddenly realised that, contrary to the 'wisdom' of the internetz, my MPG hadn't changed, the car still performed pretty much exactly as it used to. A year on I've saved some money and - astonishingly - despite feeding it 'inferior' (but widely sold and favoured by most of the motoring population) fuel my car still hasn't blown up!
If you need to examine a graph with a magnifying glass to see if there is an improvement then there probably isn't.
Geting rid of the spare tyre will make more difference.
Cars specifically mapped for 99 might be more sensitive to the difference, but stock cars are not