I don't think Payload mode is triggered by a sensor or by flight performance. When you enable Payload mode, the behavior of the drone is recalibrated to accommodate the extra weight attached to the drone.
Since the F stop number is the ratio of the lens focal length to the diameter of the front of the lens, two cameras with exactly the same size aperture/opening will have the same "f-stop rating".
Differences in lens materials, the sensor, etc...
It's more than just changing movement parameters. You are changing the actual function.
I understand what you are saying about it behaving differently to you than the controls on an airplane. They are not the same controls. When you pull back on...