Muffler/exhaust science has now advanced to the level
that you can have a factory-like quiet exhaust
with no more than 2% Horsepower loss,
and that is above 5000 rpm, wide open throttle.
Below 3500 rpm the HP is less than 1%

The vast majority of Dynos cannot reliably repeat measurements at 1% variance.

A well designed “quiet” exhaust can produce more peak Horsepower than a straight pipe,
not counting “tuned length zoomies”