Sitting the two cats side by side it is easy to see the OEM single cat is much larger in diameter allowing a greater surface area for the exhaust to flow through. Now if I was going to gut the cat I would sleeve the Magnaflow cat as it is smaller in overall size and fits nicely under the truck. The state vehicle inspection does a visual inspection on the cat so I have to at least have the body installed. I will probably stick with my basic plan and just use the single OEM cat behind headers and "Y" pipe.