other than beautification, nothing really to be gained with a test pipe or premuffler if there are no leaks in your current hollowed cat -- dremel off the bolts and remount with new hardware and your "cat" will continue to serve you
If you do want to change things up, recommend checking out M&K's active exhaust premuffler -- have the cat version on my targa and really enjoy both the sound and the butt dyno bump when the muffler bypass valve opens up.