[KMPrenger]

I don't know about the M6, but I can tell you on the A8 (in manual mode), it will hold onto 8th gear when cruising on flat ground (steady throttle) until somewhere in 40 or even high 30 mph range. Seems like it won't force a shift down until 800-900 rpm.

I might not be remembering that correctly, but I do remember trying it out a while back and it surprised me how slow I could go before it would shift down.