I just went with 2-1/4" pipes, low-restriction catalyst, and generic muffler shop turbo muffler. It doesn't really get intrusively loud until over 70mph, and even then isn't too bad. And that's with the soft-top.
There's still a decent restriction at the y-pipe junction (sure wish someone would build a nice y-pipe), and these engines aren't very big, so you'd get diminishing return if you get much fancier than that.