We built this because there was an incredible level of demand on Bimmerforums. The idea is that it will fit both 52X, 530, and 540 sedans with 1 exhaust for all of them.
Over 20 people requested we build it, we did. There is no other way to build this given the limitations of the E39 chassis.
One other point, it's designed to give 4hp-7hp just like our other Dyno tested quads.
Finally, the way it's designed, the exhaust gases come out of both sides at exactly the same time, FWIW, this makes this exhaust far different than other quad tipped exhausts out there.