My problem with that test is that the "manifolds" they are using are actually cast iron headers. So they are quite a bit better than your typical production manifolds.
As for power increase, when I replaced a set of early hi-po 340 manifolds with TTI headers on my 67 Barracuda with mild 360, there was a HUGE difference, and it was everywhere, not just up top. (Now I did also replace the 2 1/4 inch muffler shop pipes with TTI 2.5 inch mandrel bent X-pipe exhaust, so maybe that helped, too.)