so wait, were the AFR changed by the tuner or did they change from the different header?? 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. i believe this is so going from OEM to aftermarket. but some will argue that 1. you shouldnt just throw it on and run it because you could run way to lean. so tuning needs done first. 2. others will say dont tune because that changes things. you should run the car untouched. so whos right???
personally i think all tuning should be done to the max!! who cares if the car make xxx more power by just throwing it on, when it could be dangerous in the long run. tune the car as best you can with whatever header and call it a day. then put the new header on and tune it best you can just as if you werent going to tune it anymore and the car leaves for home, happy and safe. then you'll see true results as if the car showed up to a red light ready to GO.
so the tune showing OEM vs. aftermarket needs changed. either make the OEM tune show 11.1 AFR or make the aftermarket show 10.5 AFR. then there on the same playing field.