Read this...
http://www.4crawler.com/4x4/CheapTricks/EngineMods/index.shtml

Besides what info you read there, i'll add my own opinion: Get a "high flow" cat. I don't know where you got your prices but you can pick one up all day long on eBay for $50-$60 shipped. Click Here

FYI, without adding all the tech stuff....a "high flow" cat and a regular new cat are pretty much the same.