high flow catylitic converter

I have a 87 camaro with a 305. I just installed edelbrock headers and now puting on a flowmaster 3" catback exhaust. I wanted to know if a highflow catylitic converter is any good? When I installed the headers the air check valve won't fit it hits the valve cover on the passenger side. Do you need these check valves for emissions or can you just cap off the lines?Do they hurt horsepower? Thanks for any help