A muffler swap would net you more power and sound to boot. If you go with true duals all the way back which is pricey but that would get you the most power. I am looking at a muffler swap and the stock pipe seam to be 3" from the y-pipe split back.
True duals all the way would reduce power. A crossover such as an x pipe, h pipe, or y pipe makes more power than two separate dual pipes not connected.