Does 11" fit the rear? If so, match the wheel/tire specs from a Porsche Panamera Turbo S for the 20s. This is important b/c the rotational distance need to be the same for the front and rear on xDrive. The Panamera Turbo S has AWD (with 20s) so that should help get close to the correct setup. On setup, I would go with Forgestar or other custom wheels to ensure flush fitment and offset. I don't like messing with spacers.
I don't think you can use the RWD setup from the M5.