Hey Whit,
Regarding cat back systems, a few months ago a baffle in my stock muffler came loose. You could hear it squeaking from outside as it moved, and it caused the engine light to come on. I ordered up a flowmaster catback system from Summit for either $130 or $160 (I think $130) and, to be honest, I am well more than impressed. It will pay for itself in 6 months. The improved gas mileage was obvious, better acceleration, and not too loud (I don't like loud exhaust, low and throaty is good). My local muffler shops were wanting to charge $90-$110 for another OEM muffler and tail pape only. What a scam! I bolted the whole thing on in a couple hours and am really impressed. It is a single exhaust, but well worth it.