if a header makes you run super lean, then yes the headers adds power because it along with the the car running leaner added power.
Thing is, you don't need to change headers to get leaner. You could just change the tune, and save yourself the cost of the header.
It could be that the header gained no power other than what you'd gain by tweaking the tune and using the original header. I am not claiming that this is the case
- in fact I really like the design of this header - I am just saying that this experiment doesn't provide evidence that the header enabled the car to make more more power than simply retuning.
And I'm still really confused about why changing a header would cause the car to run leaner in the first place anyway. Like I said earlier, if there's more air going through the motor, there's more air going past the MAF sensor, so the ECU would be adding more fuel accordingly. It looks like something else changed, not just the header.