I agree with the NPP suggestions. I haven't heard enough of the SS without NPP but I can guarantee NPP in stealth mode will be much more quiet.
I think the car with the windows up, even in track mode, seems much more quiet than it actually is from the inside. I also think my neighbors hate me and think I'm speeding because I leave it in sport mode coming down the street and get an occasional dirty look. I think humans equate sound with speed, just look at this prank video to prove it:
I'm veering off topic though. I just don't see any aftermarket companies making a quieter exhaust and expecting it to be a big seller.
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2017 Black 2SS, M6, NPP, MRC, NAV, and black bowties - delivered 6/23/16