Mine has it (Done by Awesome GTi by the previous owner).
I am definately not the sort of person that would have ever considered "chipping" their car, but I have to say it's well worth it.
Having driven standard gti and R32, I can confirm (and the dealer that supplied the car agreed) that the car is easily as quick as an R32 (not the same traction of course).
I was expecting the car to need more revs, but the remap has improved the torque also. The final icing on the cake is that fuel economy overall is up.
I swapped into the gti from a 4.2 S4, and was expecting to be dissapointed. Clearly not quite as smooth as the V8, but surprisingly poky.
Hope this helps. Welcome to come and have a ride if in the Surrey area...
Ed.
Oh, and as for being undetectable - anything is detectable, but you have to look for it - and I shant be asking them to look. I certainly wouldnt expect a dealer to notice - it's hard work just getting them to undertake the service requirments properly!