Ditto the comment about thinking revo was expensive, the MTM pice just couldn't be justified unless officially approved by Audi UK rather than just your local dealer's good will, Audi UK will be the ones that actually approve any warranty work after all.
In regards to maps, well, I'm on my 3rd S3 but 6th 2.0 variant so have had a little experience. I have used the following companies products/mapping - Superchips/Bluefin, GIAC and finally Revo.
In a brief summary
Superchips was conservative and 'safe' with a very predictable and linear power delivery.
GIAC, a superb product, very torquey delivery, yet linear, the car always feels well within its limits and in my opinion a good selection for mainly road use.
REVO - I really never expected this to be much different to the most extreme versions of GIAC I had sampled and on first impressions (on the road) it felt very similar with a very strong low rpm power delivery, but the real differences can be felt on both the Track and the Dyno. The Revo stage 2+ software is IMO the ultimate software for these cars, it does exactly what it claims and provides superb progressive & very, very strong power right to the red line.
I have tried all of the various stages of tune with all 3 companies and in my opinion if you want a warranty backed remap go for Superchips/Bluefin, if you want to move onto the next level then you have the choice of GIAC or Revo, both are excellent but Revo has the edge with proven gains and big power.
I have no experience of QST but there is no way I would even consider £1,000 just for software, true in life you get what you pay for but I remain to be convinced that QST can offer anymore than its competitors.
Good luck with your choice, stick with any of the big players and I'm sure you will be happy, the S3 is a superb car when mapped well, I love mine