It's real easy. An optimized combo with headers will outperform an optimised combo with manifolds. The milder the combo producing lower output, the closer the gap. The lower the output, the closer the gap between lousy manifolds and really good ones. With headers being lighter, along with the power output increase, you cant loose. Now if power isn't the main concern, rules, or ease of installation. Then manifolds may be the right choice.
Doug