an aftermarket racing header will not have a cat and they are usually a thinner gauge metal than the factory so they will likely be louder. different headers can sound different depending on how the pipes merge. how they merge will affect hp characteristics and sound. however, depending on your muffler, you might not notice the difference. if you want to keep your slip on exh, just make sure the header will work with a factory muffler. some aftermarket headers will only work with their muffler set up. so your slip on that matches the oem connection points will need a header to end at the factory location.