I don't know how it's supposed to sound since the baffle was wrecked when I got the bike, but it sounds alright I guess. It's supposed to be nice tomorrow so I'll try to put up a video.
I think it's probably more restrictive than I'd like with the 4:1 that I've got; the muffler outlet is about 1.25" and the secondary pipe from the header is just shy of 2", so it's quite a step down. I think the head pipes on the 750 are bigger than the 550 anyway, and mine are 1.5" OD. Still, they're awful shiny.

Don't feel too much like a moron. We learn to work with what we've got. The bracket was just a piece of scrap I had laying in the pile, cut, filed, drilled and polished into what I needed it to be. When you have lots of time and little money you're forced to be creative.
FYI I get all my scrap from a local scrap metal yard. Go have a look around and see what you find. Not too often and never too much at a time and so far it's always been free.
BTW this is what I started with.
Has to be an improvement.
tony