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.
2014 M6 Convertible Citrin Black/Champagne - 6MT, Carbon Ceramic Brakes, Exec, B&O, Night Vision, Driver Assist
2013 750i Mineral White
2011 750i Jet Black
2007 750i Monaco Blue