Why does everybody say lowering your car w/ big wheels makes the ride quality awful? If done properly, it looks great and rides great.. just stiff. I have yet to be in a lowered car that has given an uncomfortable ride.
Also, im not sure if the m5 suspension will fit, and if it does I doubt it will lower your car because the m5 is a good bit heavier, so the springs will not be compressed as much as they are designed to with the weight of the 528 on them. Even though the m5 shares the same chassis, a lot of parts are not interchangeable between e39s.
Just look for a good suspension set up designed for your car, it will be best.