doesn't matter. If he is running a catless uppipe or not. I have no cats period on my car. I have headers, catless uppipe, catless downpipe, and running my race pipe and throw no codes. I know my high flow cat I have is not perfect. When I was at Magnaflow in CA putting one of our machines in there. I talked to one of the designers for the exhaust. He informed me when they make high flow cats they are not designed to see High temp high boost use. Hence, mine only has like 3k miles on it and the insides are starting to fall apart.
The downside is Modified Mag did a test and found you actually loose horsepower when swithing from a race pipe to a high flow cat. They also tested a cat with the guts beat out of it and found that you actually loose even more horsepower compated to that of a cat that is still in tact. They said it must have something to do with the flow. When you beat the cat out it creates a big wall for the exhaust. Only way to do it correct is beat the cat out then basicly "sleeve" the gutted cat with a stright pipe then weld to make the gasses flow though so it doest hit on either side of the cat where there is no pipe.
