Well, some members here already told you not to take it to BMW, but you said you were happy with BMW. A mate of mine has an M4 CS, and his car sat at BMW for almost a month with them changing his spark plugs 3 times, trying to figure out a misfire. I told him to take it to Xflow, and Alister took him less than 5 minutes to find out what the problem was...low coolant.
BMW techs are the replace till fix crowd. A guy like Alister, who does engine upgrades etc, UNDERSTANDS a motor, so he won't just go around replacing everything in an engine to fix an issue.
But from the sounds of it, you are insistent on taking it to BMW. I have a hybrid turbo N55 that has run flawlessly for 25k km done by Xflow and Xcede, with all maintenance work done by them. Not BMW. So have fun with the very expensive service from BMW, which will replace a blown plastic charge pipe with another piece of junk plastic, etc. I would rather stick to the Xflows, Xcedes, 357, etc crowd. To build the cars they do, they actually have to understand the engine.