Having to pull and test four different set of piping, before zero out the drone, sounds annoying and monotonous. I don't envy Dinan's engineered; they earn their keeps.
The Dinan example sounds beefy but the crux of the S55 is no matter how much tinkering and sound deadening you try to balance in any exhaust barrel; you'll never get any combination of piping to sound sonorous and monotone, instead BMW program that overrun "pack-pack-pack" in which the break up of a steady exhaust sound, shoots directly from the manifold of the turbos, manipulated by the ECU and is touted as a "feature."
All other manufactures found a way to make their bi-turbo exhaust to sound right. I don't know what exactly is BMWs game plan but it's hard to imagine then can't find a way to improve the way it sounds from the factory.