Care to consider the older Max Wedge cast iron exhaust manifolds of the 413s and 426s? Or the Ford 390HP or 427 cast iron exhaust manifolds compared to their stock log manifolds?
What comes out the end of the manifold is not a homogenous flow, but has pressure spikes, positive and negative. The key might well be how these positive spikes interface with the negative spikes of when the exhaust valve is open during the "overlap" phase of each cylinder's valve timing?
In my orientation of things, the HP manifolds are an improvement over the log-style manifolds. How much might be open for discussion, though. Size does matter. Otherwise, we wouldn't have had them for all of these years. Marketing didn't have anything to do with the Max Wedge-like pre-cursor to the HP manifolds, as they were used on drag race cars first. AND there is something of an un-optimized engine dyno test of headers, HPs, and their pre-cursor cast iron manifolds at Nick's Garage. Neither of the manifold pairs, to me, worked as well as they might of in a better-optimized situation.
Granted, some hot rod parts were designed by sight rather than actual flow bench (which didn't exist back then as they did in the 1980s and later) flow numbers and related dynamics.
Just some thoughts,
CBODY67