Yes, agreed on that also: Compared to M5 which is fine in that department, the aerodynamics of the X6M and the 1M are both SUV-like which undoubtedly affect their performance negatively after some point in speed: Look at the difference between the M5 and X6M "after" they reach 100km/h.
I also notice now the difference in tires; the X6M has slightly larger tires but not max performance category at all (Bridgestone Dueller), the M5 has the best in market for performance street driving, Michelin Pilot Super Sport; but still at launch X6M is quicker till 60km/h and this despite the tire/weight disadvantages! I guess the auto box in X6M works much better at launch
