Just came across this and checked out their YouTube clip.
http://maxhaust.de/bmw/x5/x5-f15/kom...ndbooster.html
The Hamman 'fake' system is ridiculously expensive at 7000 EUR. This is significantly cheaper at about $1500 EUR.
Maxhaust have a popular setup for the SQ5 which already has both internal and external sound. So in that case it's just a tune as Alan described above.
The system proposed for the M50d looks like it requires hardware additions, though the results are essentially the same as the Hamman system.