i had kooks high flow cats on my 08 bullitt mustang. car is heavily modified with gt500 supercharger and other mods, makes 650 rwhp.

the cats worked just fine as far as flow and no stink.
the problem was they flowed to well. the computer didn't read enough temp differential between the front and rear o2 sensors, or whatever the computer checks with the rear o2's, and thought the cats were defective. it gave me a check engine light.
the newer "green" cats from kooks are supposed to correct that problem.