I heard one on a local race bike..I think it was a CBR600. Sounded like Vance & Hines to me. Haven't seen any on a 12 though.
I would say go for it if you like the looks. Just give us a report back, and pics...
Be warry of the carbon fiber slip on pipe. I was told from Muzzy that the reason why the other guys don't make one is because if a piece of your catalytic converter breaks off and flies into the can say hello to needing a new can. However, if you have already gutted out your cat, no worries.
Iv'e had two different slip ons for the 12, and who knows how many for all my other bikes in the past. What I would like to see for the 12 is someone that could make a slip on with a mid pipe. They could include instructions on where to cut the stock pipe and then just clamp new mid pipe on with normal end can hardware.