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 : 2015 328xi MSport Silver/Red
His : 2016 X3 35i MSport Silver/Mocha