Many opinions here, most formed without experiencing it firsthand and many on a principal basis.
Have experienced in both M6 and M550d. It really works and sounds great inside the cabin. And instead of hearing all the other noises you would by removing insulation, you can actually get a better aural experience if other noises are filtered out.
As has been said, it's engine noise, not exhaust. I guess BMW has focused a bit on induction noise to enhance that part of the aural experience. Kind of similar to people adding a open air filter to get more noise (but feeding hot engine compartment air to the engine at the same time...).
Noise legislation is getting stricter and stricter and this obviously makes it harder to make engines that make lot's of outside noise. By going this route, you can have a car that meets noise legislation and doesn't annoy the public (these kinds of cars more than others has to be "acceptable" to the feds and public to avoid being banned in the future. BMW as a responsible company needs to take this into consideration as well).
Regardless of whether Porsche uses a different system, isn't it by definition just as much "cheating" that they also have to artificially open up passages to be able to get a decent amount of sound to the cabin? I mean, you didn't have to do that on a old 911... It made enough sound by itself to be heard inside
