Running your car on 95 RON will not damage your car in any shape or form. Using a low quality fuel might. There are poor quality 99 RON fuels too....
The car is designed to run on 95 RON quite happily but might not make the manufacturer stated power or mpg. The mpg difference is negligable based on my calcs on a click to click fill up.
Many many A3's and A4's with the 2.0T in them have never seen anything better than Morrisons finest 95 RON and are still ticking on.
FYI, here's my MPG figures for the last couple of months:
Date Lts Miles Real MPG Cost Cost /l
30/09/2011 39.81 319.8 36.5 £52.51 £1.319 95 RON
07/10/2011 26.14 202.7 35.3 £35.52 £1.359 99 RON
21/10/2011 38.46 291.2 34.4 £52.27 £1.359 99 RON
27/10/2011 40.25 299 33.8 £53.09 £1.319 95 RON
04/11/2011 43.05 314.5 33.2 £58.94 £1.369 99 RON
17/11/2011 44.86 343.6 34.8 £60.52 £1.349 99 RON
21/11/2011 41.78 325.5 35.4 £56.36 £1.349 99 RON
25/11/2011 33.58 251.9 34.1 £43.62 £1.299 95 RON
07/12/2011 34.71 240.8 31.5 £45.09 £1.299 95 RON
14/12/2011 43.14 319 33.6 £58.20 £1.349 99 RON
As you can see, changing from 95 to 99 RON makes little to no difference to the real MPG.
There is a noticeable difference in performance, more so when you're using the upper part of the rev range than the lower part...