well i agree that things can only flow as good as the smallest point but that only works in the intake or exhaust because the when fuel and air combust in the combustion chamber it creates more gas that has to be pushed out the exhaust this is why you see a 3 in inlet on diesels and a 4 in exhaust. It will help with a cat back might not be much but it does cause restriction. for example go blow in a pvc pipe that is one inch then go blow in a 2 inch it is much easer
