I've used Statllers and JBS extensively and both are excellent tuners.
JBS have a TFSI Leon as a demo/test car. They develop their own maps and they are very smooth and progressive. Until recently I had Custom Code on my car and it was very smooth, but ultimately didn't give the outright power I craved
Statllers have 2 ED30's as demo/test cars - one manual and 1 DSG. GIAC use them for testing their beta maps, so they are very experienced with the K04 limitations. I'm currently running GIAC on my DSG box, which is a must if you have DSG.
In Sheffield Swifts are the registered Superchips/Bluefin dealers and I've used these in the past too. Very well established, but deal in all sorts of motors and not specific to VAG....still decent though.
HTH.