pez, I can't write from experience since unfortunately none of the tuners have a package available for the newer engine that Audi started installing from mid-2014, however it is my understanding that MRC do have a genuine technical advantage over the others (although if this is not true I'll gladly be corrected). Now whilst both APR and REVO are excellent tuners, theirs are off-the-shelf tunes. They are designed to be a standard package that can be installed by distributers without necessarily having tuning skills (and don't take that as a criticism). However MRC offer custom tunes. I believe that their stage 2 package includes air intake mods that are not always included by the others. But importantly as part of the package they make sure that the remap takes best advantage of the hardware mods there and then on their rolling road. They notice things like exactly how good the fuel is in the car at the time, so it is clear that they are tuning YOUR car and not just another S4.
So again, nothing against any competitive tuner, but there is a tangible difference in going to MRC.
