I'd think the deflector (was a solid black piece of metal in my E65) would be designed to carry the wind stream over the roof opening, but maybe they found that it was just directing it into the face of rear passengers. The diffuser is meant to just break the wind up - sort of like those rear wind screens that some convertibles have. I had some type of chevrolet as a rental car a while ago (i think it was an impala), and it had some loud diffuser also. I wouldn't imagine BMW would use them because they're cheaper than a piece of solid metal, but who knows with these guys.
2011 750i Jet Black / Ebony Interior / Black Ash Wood Trim
Factory Options: Luxury Seating Package / Premium Sound Package / 19" Alloy Radial-Spoke Wheels / Rear View Camera
Aftermarket Options: 30% Ceramic Tint / PIAA Xtreme White Angel Eyes / DVD In Motion