IMO the V12 760i must remain the 7er highest number or it will lose image and credibility.
Logically the M Performance version which would slot in between 750i and 760i should be called
M755i, but if BMW's marketing considers this number to not look solid and attractive enough (no zero at the end...) than yeah they have no choice than to call it M770i... unless the 760i becomes 770i so that the M Performance version can be dubbed M760i... But the -60 has so long been associated to the V12 engine that 770i seems highly unlikely... who knows.
I guess the less bad and most logical solution would be M755i... which still sounds ok, but what if BMW decided to make a similar version of the 5 Series (along with the M550d), it would be called M555i... a bit weird