Revo is a company which supplies re maps for all VAG cars. It has a very good reputation and if you purchase a SPS device (about the size of a packet of cigarettes with a dial on it) you can alter things such as types of fuel, timing, boost and even go back to standard, just by plugging in to your car. There are 3 main SPS to buy and it depends how much you spend as to what you can change.
I would suggest that you pay the extra as you have put the idea in their minds..
and they may just check it..
The others are right in that it is hard to check and would be too expensive for them to send every ECU away for checking..
You also have to tell them if you change other things such as exhausts and alloys as these can be seen by any old assessor..