Couple of things: First, the factory tin head gaskets are only about .015 thick compressed. Your typical over the counter fel pro head gasket (like I used in the Summit kit) is about .050 thick compressed. So, as you can see, .040 is only going to truly net me only about .005. Now, at the other end of the head gasket spectrum, if you go with a .028 compressed performance gasket, you are still only netting about .030 from the factory.