i've heard the same thing, don't know if its true or not. i cut my cats off, i don't know how much HP i got from doing so but i'm sure it was some. alot more sound with no cats too. and i turned the back O2 sensors off with my sct tuner.Removing the Cats causes the check engin lights to come on. Have heard this causes the engin to run lean because it thinks the cats are broke thus killing your gas mileage. But most tuners will eliminate that, or get the mil eliminators, also shuts off the check engin light.