You've probably found one by now, but I have previously taken my C63 to AMD Tuning in Thurrock, Essex and the experience wasn't great. They're good guys but they really didn't seem to be able to 1. strap down my car enough to stop if from spinning its nuts off and 2. enter it into dyno mode so that a run could be made.
The result was that my car dyno'd in Comfort mode only and up-changed the gears as it felt appropriate for that mode (i.e. as early as it felt possible at the present pedal position), and the graph was all over the show what with all the wheel spinning and gear changes. I didn't really know what to make of the experience, as even with a run only covering 2/3 of the rev range it made good power, but clearly wasn't a clear representation at all. They didn't offer a refund :-/.