If there is a //M5 in the pipe line, and thats where your heart is going, may I complicate things completly with a bit of logic.
Apart from the fuel costs, part replacement is scarce, and very expensive.
The E39 //M was sort of managable, although high, the M5 is a killer.
If your going to go that line, get a 550i in manual (not as scarce as jou may think) and work it a bit.
I know its not the S85, and that does make me a sad Panda, however its more logical, oh and the car costs half of what the M5 does as well.
I am also not just referring to the gearbox issue, EVERYTHING is expensive.