Max, the smaller ID stock head pipes help low end but can't breathe with a full exhaust up top. Apples and Oranges.....
I am just going by information I gathered from others, that even at peak RPM, the muffler is not a single restriction point because entire exhaust system (headers, cat, pipes, and muffler) is matched. I have no direct data, but I remember it seeming reliable at the time. I wasn't speaking to an entire exhaust and header replacement, that is a different story (which I did mention in the last post, but perhaps it wasn't clear).
In the way back, bikes often had a restrictive muffler (poor design, probably just to pass EPA at the time), and that ended up causing a considerable single-point loss of power because it was overly restrictive for the rest of the system. That is not the case on the C14. Replacing just the muffler makes only a slight change, it isn't the "must have" like on the old bikes (technology improved, which is not a surprise). Loud is not power
Someone else on the forum was obsessed with the CAT, thinking IT was restrictive and actually cut it out. Nope, no significant change in performance. It did run a little cooler near that area.... and the bike was then highly illegal
I believe most swapping out of stock mufflers with aftermarkets is also illegal, but I think that just is somehow accepted (not sure why, it is certainly visible enough) but it also doesn't cause major emissions no-no's. Of course, reflashing the bike or using a PVC is ALSO illegal... but it is also not easy to spot and, overall, with a good tune, can actually not harm emissions much in the "real world." On mine, my even MPG went up, overall.