I have the large (20" long case) Dynomax SuperTurbo mufflers and they do a decent job of reducing exhaust noise and did not seem to hurt performance.
I have the 3" in/out mufflers that came with my TTI exhaust kit on the 500" stroker (near 700 HP?). With the 3" size and big engine the exhaust is still fairly loud.
I also had the 2.5" versions on my 360 RV engine (330HP?) and they were fairly quiet till you got on it.
The Super turbo mufflers do as stated, pretty quiet without much if any performance change, but they don't have a really nice sound/tone in my opinion.
I have heard the Hooker Aerochamber mufflers on a friends car, and I really don't like how they sound either. I don't know any Magnaflow muffler cars for comparison, only my diesel truck with 5" exhaust
I have a chambered IMCO muffler on a 318 that sounds nice, but not much power flowing through it either?