For what it's worth, this week I measured two new, aftermarket Burgess-type silencers on my flow bench:
Burgess-type #1, 90.9 CFM
Burgess-type #2, 184.4 CFM
For comparison, an OEM semi-megaphone from a Clubman Gold Star flows 186.5 CFM (all measurements at a pressure of 20.4" H2O). I don' t know what either sounds like but I suspect there might be a difference. I know there would be a difference in the h.p.
Two years ago I put a new shorty silencer on my Catalina Gold Star, but initially it ran horribly. The problem was revealed when I removed the air box and saw a cloud of fuel at the inlet of the carburetor, ejected by too-much back pressure from the new silencer coupled with large valve overlap. The cloud enriched itself as it passed over the spray tube on the way out, and enriched itself even further when sucked back in. Drilling a number of holes in an internal baffle fixed the problem.
Related to this, even with the restrictive silencer the bike ran fine when the air box was off (at least when revving it on the driveway), apparently because enough fuel was able to escape from the cloud so as not to be extraordinarily rich. Before I discovered the problem and drilled the holes my wife commented that the bike didn't sound right.
The exhaust system flow does affect the h.p. but it's not a simple tradeoff of h.p. vs. decibels. The frequency spectrum matters to how annoying it is, and some do a much better job than others in killing the offending tones without restricting the flow too much.