Not to throw water on anyone's parade, some guy from another board that went defunct put an aftermarket cat on his car. Ended up getting a CEL. Tried fruitlessly to get rid of the CEL, ended up putting his stock cat back on, and the CEL was still on. Damn, sucked to be him.

 

I have a random cat on my car and it works great. I bought it new in the box off of ebay for $60.00. I think it freed things up a bit but I'll never know for sure how much HP I got from it.