The engine idles at 660rpm (is the n55 suppose to idle at 550rpm?) according to ISTA to but it does have a occasional +-50rpm shift which apparently is normal, With AC the surging is more pronounced, also the car has light pulsing when in gear and brakes applied I think NIC (Neutral Idle Control) is not engaging.
It should be 550rpm, so same as for the N54. You could check the index of the injectors. Old versions are often unreliable. As they are super expensive I wouldn‘t recommend changing them without further investigation.