Hi-flow cats

Hey guys, so I have a aem short ram and a dc 4-2-1 stainless header due to come in a few days and the guy at the shop also recommended a hi-flow cat, Im just wondering if it will give me a considerable gain over my stock, im running with a skunk 2 catback system. Also arent there some legal hi-flow cat's out there? Just to make it easier instead of swaping it out everytime I go for an inspection.