Shall we do the floatation or flammability test first?
There may be something to say about a aftermarket exhaust. The design of the stock ones is often very related to emission testing. A cat will warm up faster if it's smaller. Most states that do emission testing only care about a warmed up cat, so you could legally open things up a bit. But a muffler isn't going to make a heck of a lot of difference. Almost all after market mufflers are made to look cool, make nose, and that's it. Some of the better ones will reduce exhaust pressure, but most stock ones are really quite good. Also the stock mufflers are good at muffling.
If you want a muffler that is bad at muffling go for it, but call it by it's real name: Fart Can.
That's an interesting thought about cats warming up faster. However, I still don't think it represents any loss to people who drive for fuel economy...we're just not flowing that much exhaust.
Anyway, modern converters are pretty much free of any obstruction.
I was just starting the "wild accusations".
If you drive in high gear with WOT you do flow a lot of exhaust, not as much as you would in low gear, but still quite a bit. But then again even if the exhaust is restricted for this driving style a freer flowing cat wouldn't necessarily mean any improvements in FE. In fact a more restricted exhaust may be helpful. I think the best bet would be to stick with the stock one designed by an expert.