I think that manifold/header test is flawed. I base it on the fact that Holdener on YT tested a 300hp Crate Motor with the dual plane M1 intake against a JY 5.9 with a beer barrel intake and got virtually the same numbers. If dropping that extreme runner length intake won't pick up power, I don't think much will.
My opinion is that the real restriction on that motor is the cam, and until a better cam is swapped in the real advantage to the manifolds and/or headers isn't going to show itself. Not saying there weren't advantages shown, just that it was less than what it would have been if the exhaust had been the restriction.