I'm running "true duals" with an x-pipe on my '05 with no problems. I'm not sure if I've gotten any performance improvements, but it sure sounds hella good!

I took mine to a custom shop (Henry's Mufflers if you're anywhere near Raleigh, NC this is a great place to get work done!) and they cut my exhaust after the Cats and before the Y pipe, then ran the exhaust pipes down to an x-pipe, then into two SISO Flowmaster 40's, then straight out the back. I love it!!!