Ok guys will it affect my cars performance and or gas mileage if i bypass the cats. Will this affect my o2 sensors in anyway and also the person before me dualled this car from the manifold all the way to the back will that make it louder too?

 

since you say dual im assuming you drive an sc400, if you put a straight pipe in place of your cats you will get a CEL. so you go to autozone and buy 2 anti-foulers for spark plugs, drill the inside of one out to fit the o2 inside and then screw those into the o2 port, then the o2 sensor last. so it is taken out of direct exhaust flow, gets rid of CEL's. however, i have both of my cats in place and did a 3rd cat delete with straight pipes and its crazy loud, sounds good but loud, i cant imagine how loud all 3 cats deleted would be. remember on an NA motor, if you free up the exhaust flow, free up the intake air flow(k&n filter at least) just my 2 cents. you would see some mild HP gains, nothin to call home about.