I have heard "car guys" from all different backgrounds say single exhausts make more power, then again, I have heard people at exhaust shops and other car guys (but not as many) claim there is no loss in a dual setup. I think there is no reason to go single on a MS6. You already have a nice exhaust and those tips look sweet! I wanted a MS6 but could only afford a loan on a brand new car and since they didn't make the 6 anymore I got the MS3.
Side note, not trying to thread jack, thinking it might be unique to go dual exhaust on my speed3.... does anyone have any evidence to support a different sound from dual exhaust or a quieter exhaust from having two tips??