On a rather mild build you could get it where there wouldn’t be much difference between headers and manifolds....... but as the power goes up, the spread between them will grow.
A couple years ago a guy on Moparts had his TF headed 6bbl stroker on the dyno.
The intention was to run ex manifolds in the car....... until they bolted a set on the motor and saw the power numbers go down by 90hp.
