i'm interested in switching out my exhaust on my stock '99.5 2.8. I've heard that placing a cat-back on a NA engine might reduce low-end torque, so I was planning on just switching to a new muffler. Would this give any performance benefits at all? Also, would switching to a high flow cat while retaining stock piping have any benefits? I have also heard that 2.8s have two cats, but I might be misinformed. How would I go about switching two out if there are in fact two? Thanks for your help and suggestions.

You won't be switching out the cats easily on a 2.8L car. The piping for the cat starts at the exhaust manifolds, and goes into a Y pipe (its a V6, so it has 1 cat per bank of cylinders). You'd have to cut the old ones out, and weld in some flanges, so you could bolt in some new cats (its easier to bolt/unbolt them than it is to cut and weld each time, may as well do it right the first time).

As far as mufflers, a Dynomax SuperTurbo would be sufficient if you used two, one per bank, in a 2.25" diameter, or a single 3" (no other mufflers in the system). I'd recommend just doing a whole new system from the cat-back, it'll be easier to put two mufflers.