Oh, this seems very logical and does make sense as far as the 17" rolling diameter against 19" rolling diameter. I tried this after I woke up from writing that...damn nanna naps! I happen to have two new tyres on 9x19" and one ? x17" and they roll exactly the same.
Yes, you are correct in what you said about the transfer of power bit...It did say that the Xdrive can transfer all the power to the one wheel if it was necessary. Guess you can;t believe everything BMW say
Cheers mate. Jeff