I think we've already seen the tri-turbo experiment or derivation from a previous m-engine - the tri-turbo diesel engines in the X550d and X5d. Also
the development of the 1M helped BMW stress the efficiency and resilience of a FI engine on a track.
The sound of the F80 M3 in that video is definitely I6. The engine will combine direct injection, valvetronic and three turbos (small, medium and large) to
simulate the linear power curve of an NA motor (similar to what we have seen with the tri-turbo diesel).
V6 would be an interesting move, but I don't think it's as easy as chopping
two cylinders off. Plus they sound like crap (go listen to a Nissan or Infinity