the reason why stock cats are restrictive is because the flanges on both ends are usually smaller because they were made to fit a stock exhaust, for example cat flange size on stock cat on a car say an integra the size at the flange will most likely be around 1.75' and when you have that stock cat on an exhaust and header with say 2.5' tubing and 2.5's at the flanges on both header and exhaust then you are going to have a bottle neck effect which slows down the flow velocity which creates restriction which leads to backpresure which is bad. the 3 parts to the cat are the reduction catalyst, oxidation catalyst, and the ceramin honeycomb catalyst which are all free flowing especially in hi flow cat converters. the only reason why you get gains from a test pipe is because the tubing and flanges are bigger to match your set up. if you buy a hi flow cat such as carsound they come with the proper size flanges for your setup hence you gain more power and dont pollute the air we breath.