While BMW's S58 is a vast improvement over the S55 in the sound department, there is still much room for improvement. Most of the restriction comes from the factory G8X center pipes, as they house not only a pair of secondary cats but also a pair of very large resonators. You read that correctly; not 1 but 2 large resonators accompany a pair of secondary cats. When compared to the resonated Eisenmann center pipe set, the factory center pipes are 10 lbs heavier.
Eisenmann center pipes are designed to work with the Eisenmann Performance Exhaust for the BMW G8X M3 and M4 but will also install with the factory rear section. Available in either resonated or non-resonated versions, the center pipe set allows for bolt-on installation of the Eisenmann G8X Performance rear exhaust, converting the Eisenmann rear system from a slip-on style more familiar to European enthusiasts to a full bolt-on system that U.S. enthusiasts demand.
On the other hand, if you're willing to essentially convert your factory rear muffler to a "slip-on" style, the non-resonated center pipes have the added benefit of including an x-pipe in place of the resonator to further tune the exhaust gases and pair well with the stock system, transforming your stock muffler into a much more aggressive sound for minimal cost and will save a chunk of weight as well.