Rumors abound that the updates will not just be cosmetic. It makes sense for the F15 X5M to get more power than the outgoing model and for the X6M to share this drivetrain or else the facelift amounts to nothing more than cosmetics which will not entice buyers as much as a more powerful motor. With an increase to 22 psi (from 17 psi) as well as VALVETRONIC it is conceivable that horsepower in the ~600 range is achievable but this seems to be optimistic.
What is more likely since power is going to all four wheels through an automatic transmission in the X6M is that the facelift will match or slightly exceed the F10 M5 and F12/F13 M6 horsepower rating considering the already high 22 psi of boost for the S63TU and also offer an optional power kit to raise output further. Why would BMW give up more HP for free when they are already charging for the bump in the M5 and M6?
We will have to wait and see what happens but keeping the S63 V8 in production simply for the X5M and X6M when all other M V8 models have moved to the S63TU does not make much sense.