That's strange, I thought all e63 had the RPA system? This system does not have its own sensors but relies on measurements from each wheel speed sensor from the ABS system. It can determine a change in rolling radius from this & therefore flag this change as a drop in tyre pressure.
During the calibration stage it takes measurements at different speeds, as the diameter will be different as the speed of the car changes. That is why you see the system message "initialising" when you first reset it iDrive.
Have you reset it since you fitted your new M6 wheels? It's under "i" -> "Vehicle Settings" -> "RPA". You must do this after any change in tyre or top up of tyre pressure.
(Tyre = tire)