I ran a hitch for the last year and a half, finally removing it the other day. I was tired of the odd little rattles here and there while running a stock muffler. Even with Kartboy long hangers, I still had some WOT noise. With a smaller aftermarket can, it would have been perfect. I'll probably still go that route this spring.
Anyway, I had some mounting issues with the 06, and it's rear diffuser. Using a soft mallet I had to push the diffuser mounting brackets out of the way as it comflicted with the hitch. I also had to modify my diffuser to make room for the hitch reciever.
It was a blast driving the STi into the mountains, then biking down.