Be careful what you wish for with the 8-speed tranny. I just got 2013 335xi, and I have to say that 335 was better paired with the 6-speed tranny (there is enough tourque in the engine). The gear switching it too frequent to my liking and car feels like accelerating slower than my previous 2010 335.
I suspect same will be in the future's X5d - the 8-speed tranny will give you better MPG on highway speeds, but worse acceleration.
Hers : 2013 335xi Imperial Blue/Black with lots of stuff...
His : 2013 X5 35d Deep Sea Blue/Black with all packages
Previous : 2010 335xi Monaco Blue/Black with just about every option...
Previous : 2008 X5 4.8i Space Gray/Black with just about every option...