That's my exact setup!!! I have 2.5 until the stock rear split, and a SHO ypipe w/ cat. I did the ypipe first, and about 4-5 months later the 2.5" piping. The piping made a nice difference, but I'm not sure if it's as much as the ypipe. I eventually pulled out the two rear mufflers...
I love it. It was a little too quiet w/ the two stock mufflers in, but I live at high altitude. I recently went to lower elevation and my car was considerably louder.
