Performance loss from headers to HP manifolds

Now that moper mentions it, I remember reading a write up on a 340 years ago that went from the manifolds to 1-5/8 headers with a disapointing 25 HP gain.
Recently, (1 or 2 years ago....is that recent?) Mopar muscle did a thing on header types.
Comp Cams has a small line of cams for use in engines with exhaust manifolds. The duration specs are wildly long for the exhaust manifolds restriction.