I've done it. I wanted to trim weight slightly, and I prefer single twin look rather than the double twin pipes (quads) as it looks too cartoon-y on our cars, kinda like Mario Kart...
See here:
http://forum.liberty.asn.au/viewtopic.php?f=55&t=31010My exhaust changed for the better (beautiful H6 sound), though suspect it was more due to the work done before the single muffler (headers, hi-flow cats, 2.5" piping and mid muffler) than anything else.
I used an Ultrex twin muffler (had to buy as part of Left and Right set) which I wouldn't recommend as about 1/3 of the internal space is taken up by a y-splitter that gives the twin pipes. If I was to do it again I would go another muffler, prob Magnaflow, with the split added by an exhaust shop after the muffler - the space is there between the muffler and the bumper to do it. The Ultrex tips are perfectly placed, so it does look good, but would rather have a muffler than worked better.