Sounds like a bunch of butt dyno's at work here.
Leo Vince with the insert is the same dB as stock.
I don't believe that , nothing is as quiet as stock , nothing !
It's the same design as many others fiberglass packers , how could it be the same as stock ?
I was looking and found a set of used Leo Vince ZX muffs because it was said how quiet they were till I heard them with out the insert, that is why I got the Two Brothers and the smallest insert of the four that they make.
Have you ever saw how restrictive those Cats are in the header pipe
, when I saw how small those holes were I had a welding shop cut the collector open and remove them , They are ZX headers but I believe the stock header has the same, I did not check the stock header, I'm sure they help with the sound but believe me between the cats and the stock monster muffler they hurt HP over 5,000 rpm by 5-6hp easy.