This video is one that will give you a general idea of what you gain with headers vs. manifolds. While your engine is not the same, it serves as good information none the less.
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.)