Cats
 

It looks like I need to replace the original catalytic converter on my 2004 RX8 and I was hoping to save a little money and increase the horsepower while I was at it. But I live in California and need to pass a smog test.

Any suggestions on aftermarket cats that will do all three of these things? Or at the very least two out of three?

hey TrojanKnight, welcome!

stock cats are pretty expensive so I suggest going the route of RP High Flow Cat.
I believe people in CA have used it and sucessfully passed a smog check. (just hope they don't look under the car, they might notice it's not OEM)
http://www.rx7.com/store/rx8/rx8exhaust.html

How many miles are on your car? I know in NJ the cat is covered under warranty for 8yrs/80k or 100k miles.

^^ very good point!
I think federal emissions is 80k.

So take your car to the dealer and have them test out your cat

I'm definietely over 80,000 miles.

go for the rp supercat

i belive the cat has a warranty of 100,000 Miles.