I don't know how long my header has been cracked, so I can't really judge if it has effected my performance. I know my fuel milage is worse than a lamborghini's, and I know that my top speed is half the speed of smell, but both of those could be caused by the lift and the roof rack. I know that I had phantom "check engine" lights, although they always went away without me doing anything, so I never thought anything of them.
Either way, I know it isn't good for the engine, not to mention it can't be good for anything else under the hood to have that much hot gas pumped under there.