Unless something has changed, you should have a 4" exhaust from the turbo back. So unless you plan more h. p. and want to go 5" you may wish to rethink replacing the exhaust.
Do you tow? When I researched changing the exhaust on my 05, consensus was that if you tow heavy you will likely end up with a drone at speed. Some vendors have suggested to add a resonator in line with the new muffler to kill the drone. It was also suggested to leave the cat in. I felt that if I had to add a resonator in line with the new muffler, what's the point.
But opinion has been that the stock muffler is the restriction point and if you change anything it would be the muffler. Hope this helps.
So as of now I still have the stock exhaust 100% in place.