Might sound like a ricer idea, but would you consider running some sort of dual exhaust? You could run twin 2.5" mufflers kind of like what I'm running on the bean, and you know how quiet it is.
Otherwise you could try something like a Dynomax Super Turbo muffler. Obviously not going to flow like a straight through, but a lot of guys in San Diego ran them back when I lived near there. Since they're so strict on modding/emissions, it's what they did to keep the noise down, but still have something that flows ok.
http://www.dynomax.com/mufflers.php?muffler=superturbo
The DSM that ran in SCC's car challenge back whenever that was (10year ago?) ran one.
http://dsmporn.com/