So I would rather not have cats but I have to. I will be doing full 2.5" mandrel system with Resonator and Borla mufflers. I really dont have the cash for Cats right now but what I want to know is are they as restrictive as the Resenater? If I have to change them, what are some cheap ones that flow well? Anything else you guys would change? would you do down pipes? Thanks for the help.