The muffler is unique that it is a center input with outputs at the sides, not all many aftermarket mufflers are available in that configuration if you want to keep the stock center muffler. Usually cat-backs goes with one muffler and a T-pipe after the axle for max loudness.
Vibrant makes a muffler in that configuration in 2.5", 3" or 3.5" piping. But with so much noise absorption packing, it's going to be relatively quiet also.