Slowy, for me, my system goes 4-into-1-into-cat-into-1-into-4 and then into the 4" sidepipe, so the cat is under the hood, but you see 4 pipes coming out the side in traditional cobra fashion. So my "headers" are actually really short, and by result are actually pretty much the same length. You can see a picture here before they were coated
In practice, its going to make absolutely stuff all difference if you have some a couple of inches longer or shorter.