I agree that it depends on mods, i have a hi flow (hi flow front vg30 rear) blitz fmic, jjr split one piece dump with a 3" the rest of the way, apart from the cat, that was still the factory 2.5", put the x-force ceramic cat in today, got rid of a mis i had (although i also did the cam belt no idea if that would have helped) power is now more crisp, boosts a little earlier and harder and is also a little quieter when idling, i would say they are a good thing, that being said my next engine im hunting for 300-350rwkw and will probably upgrade or remove the cat completely.