Add a whole buncha catalytic converters, and mufflers to it? Shoot man, I'm not a mechanic. Your best bet is to research and find the quietest muffler. Talk to a few exhaust geeks and find out if a smaller size pipe will quiet it up. Stock, this is the quietest car I've owned. Before this, all of my cars also had stock exhaust. Good luck man.
You aren't likely to find anything even quiter. If you want something just as quiet as stock (or quiter) at idle, but with a better tone under load, check out the options from Tanabe. If I hadn't gone to a full Magnaflow cat-back for the wife's car I was considering the Tanabe Touring Medalion.