in regards to the duals, i didn't like the "fake" dual set-up (remus, eisenmann, etc) and it look ricey on a M5 diffuser. the other big companies that have robust R&D such as meisterschaft and supersprint uses duals. check out these links:
did you watch the last 2 videos of my post. the gearz guy explained the difference type of exhaust and their characteristics. i am so happy with my vortex resonator especially when i installed the JB4.
i bought a slightly used 535 aftermarket exhaust. if im going to do it again, i will look for a used M5 exhaust with the vortex as my resonator. i think the issue of the other F10 N20 guys is that they cut the OEM resonator and fabricate a y-pipe. this i dont recommend.