I know I'm a little late to the game here, but I don't think the Technician did anything horrible here. It's pretty standard practice by many shops to drive a car both before and after work is done. I personally would want my differential warmed up before changing the fluid. Did he drive it harder than he needed to? Yeah, but I wouldn't say he was being abusive in any way. Was he a little bit irresponsible with the way he was driving? Sure, but no worse than the way I drive my own cars. Maybe I'm a little biased based on what I do for a living, but that's my take.
I'll also add that any time I diagnose and/or fix a driveability or performance problem or do any work that involves spark plugs, ignition system, etc., I run the car hard through a few gears afterward to make sure it's not going to misfire or have any kind of boost-pressure issues. Yeah, something like an M3 or M6 is going to generate some speed and sound doing so (much like a Camaro SS). Technicians are usually very limited in the location and time they have to test a car.
Again... I don't think he did anything awful. Certainly nothing to get fired over. This is my own personal opinion again -- but if you're really that protective and paranoid about your car, work on it yourself. I paid for several things out-of-pocket and fixed them myself on my Corvette GS when it was under factory warranty. I did this so I wouldn't need to be paranoid and anxious about someone else doing a poor job, scratching it, or otherwise damaging it. If you're really OCD, that's the solution. Another would be letting the service advisor know up front you don't want anyone road-testing your car.
My 2 cents...maybe more
