we still get the twin pipes over here on the e39 m5. the 540 has shit in the way underneath the car specifically the spare tire well that makes it hard to get quad pipes. i have seen it done but it requires some odd routing and some modification to the underbody. the "m5" bumper that paul is showing you is just a look alike made for the other non-m 5 series .
by the way i think you should do quads like the one guy did with his 540 up here. Paul you know who i'm talking about. he ended up using a crossflow flowmaster muffler designed for the camaro but that thing sounded absolutely brutal and wasn't too loud when he was just cruising around.