A "ported" manifold may flow better than the mustang header but I agree with Bacondoggy, I can't see why they would flow worse than stock. By the way, I don't have any hard numbers, just speculating. Of course long tubes would be better, and the work required to get them to fit may not make it worthwhile if you can port the stock manifolds.
Here is a link to the post, Dave did a bunch of work on this.
http://www.sccoa.com/forums/showthread.php?t=84828&highlight=dave+exhaust+dyno