It is interesting to see all these heated discussions about break-in (another similar topic is frequency of oil changes). Maybe we should just write a letter to BMW and ask them to put this to rest?
Here is a piece of info I got from the primary source - when I was picking up my 335xi at the factory, it was one of the first US-spec 335xi delivered, and there was a bit of a gathering of the BMW people in the delivery center to show me the car and talk to me why I chose BMW, why XI and so on. When I said I was sad that I will leave Germany before the end of break-in period, so I cannot try it on Autobahn full speed, they looked genuinely surprised, as if I said I believe in Santa Claus. Then they told me not to worry much about break in because each engine goes though so much testing off and on the car that it's broken in already, and the procedure described was more of a precaution. Also, they mentioned that this car has been to well over 100MPH many times even before it's mileage started counting. The only thing they mentioned needed breaking in was brakes - no very rapid or prolonged braking till pads bed in. Also a soft recommendation to wary speed for first few hundred km. Not sure how/if this applies to M, but likely it's a very similar situation - I don't think testing of M engines is any less rigorous.