i don't like the idea of twin pipes - on a vee engine yes, but not a cossie.

BUT, i suspect that a single box will not keep the noise down low enough.

can't you have the normal single box for when you use it on the road, and a take off with a closing flap near the back so that for track days you can bolt on a second silencer? it doesn't matter what it looks like on the track, but on the road you want it more standard looking.

some kind of system like the aston vanquish has. when you're pootling around at low load it is very quiet, but when you start ripping it up a flap opens in the exhaust and bypasses some silencing to mae it louder and more tasty.