With a stock engine very little. They are out of breath with the kegger at 4500 and have a tiny cam that doesn't help either.

If you have cam, replace the kegger with a/m intake, etc then it might be worth the switch. If you can find some v8 magnum 92 or 93 year model exhaust manifolds they flow slightly better than the rest. I would not let searching for them hold up a project though as it is minimal difference.