The question of drivetrain loss is interesting in this kind of setup where the front wheels drive is disconnected electronically. I guess it depends on where the disconnection occurs. There is still the 'drivetrain' loss associated with the two drive axels going into the front differential and the rotating mass associated with those shafts. So it depends on where the disconnection occurs. And how that is accomplished.
So I suspect the differences in "drivetrain" loss at speed (i.e. going from 60 to 120) probably is not that different from AWD mode to RWD mode.
I'd like to see what the actual anatomy of the AWD system is before trying to determine if much drivetrain loss is recaptured in RWD mode. The Audi AWD systems are 100% mechanical and can move all torque to one wheel (front or back) based on traction. The powertrain loss in the Audi's is always the comparison issue when comparing "power at the wheels" in pissing contests but the fact remains, the Audi RS6 was able to put down the power better then the same year M5 because of AWD. And in that year 2003, the RS6 was the king of the hill. What happened since then? Mercedes and BMW adopted two things: AWD and Turbo's.
.... Old tech for Audi (and even Subaru).
I suspect there is not much difference in powertrain loss in the M5 system with the AWD off, even from starting at zero. AWD rocks.
Mike