2005 Ford Crown Victoria New exhaust system

Hello I have a 2005 Ford Crown Victoria P71.I'm thinking of getting a magnaflow exhaust system for it. The car has 4 catalytic converters, 2 on each side. One near the headers and one near the rear of the exhaust system. Would it beneficial to the performance, if I replace the stock cats with magnaflow cats? The exhaust shop guy told me that it would add more power if I replace the 4 stock cats with magnaflow cats as they are "high flow", along with buying the magnafow system for my car.I'm not sure if this is true, or if they are just trying to sell me cats that I don't even need. It would be greatly appreciated if anyone can tell me weather I should get the cats, or just get the exhaust package and leave the stock cats on. Thank you.