When I was a kid we didn't have a flow bench but we did have a dyno. Maybe there isn't much difference between a mirror finished exhaust port and a mirror finished export that has been ruffed up by a bead blaster. I have never done that test. But I do know that a mirror finished export over most stock finishes makes a big diff and keeping it spotless makes all the difference. The exhaust port sees most everything twice and therein is the difference IMO. That and flowing exhaust very well might not show the advantage for everything going on in them velocity wise. The other ports don't deal with sonic and supersonic. You can't reproduce that with a flow bench.
Of course a good pipe over a crappy one will make a huge difference but that is beside the point.