There is a lot of talk around the forums that cars are being defected for having these on, and a lot of people are having problems with the butterfly jamming after a period of use !
Fox if I was you I would be swapping to an Oval muffler so you run a bit quieter with out the need for the silencer - have a look at the different mufflers on cars at Rexpo and see what sounds good to you and go from there.