I might be wrong but AS is a luxury or "convenient" item like the auto tranny or auto cruise to make maneuvering the car in the car parks easier, and has nothing to do with performance or handling.
The 320i Sport don't have AS and u can make u-turn in 1 hand while the 6-potter non AS u find it not nice, maybe the 6-pot magnesium engine still a little more heavier than the 4-potter.
Best check with SC, try Ingress because they "mod" cars too.. :wink: If they can't do it, nobody can..
Cost? I have a friend that bought an earlier batch of the E60 which do not come with Bluetooth interface, So he got a local accessories shop to install an aftermarket unit for him. During the process, the iDRIVE system was fried. He had to spend RM30k to replace the iDRIVE system at the SC (warranty void due to aftermarket temper).
As for AS add-on, worst case scenario you will be looking at replacing the steering rack and column plus adding sensors, controllers, actuators and circuit boards. I say no less than RM30k.