Every dealership is going to be different. I had a SMG problem with my E46 and the BMW Field Service Engineer (yes, the "expert tech" from BMW NA) was trying to blame every aftermarket part I had on the car (from the Dinan exhaust to wheel studs) for the cause of the problem. Reason being that they had no idea what was wrong with my car, and it was a $6-7k job to replace the it as BMW AG recommended.
For every forum member that says "Magnuson moss", try speaking from experience (like me) before saying they can't deny you coverage. Guess what? BMW basically said my car warranty was pretty much null... Until I went to arbitration and they lost. In the meantime my car wasn't getting fixed for free.