Your comment that, “when the airplane is not in ground effect, the trim tab should be “overpowering””, indicates that you do not understand what I am saying.
Not at all. I probably should have said that when the airplane is at it's much higher flying speeds, often 2 to 3 times the landing or takeoff speeds, a trim tab that is effective at the slower speeds is going to have even more effect. And if you think about it, a trim tab that's producing enough trim at takeoff speeds is going to cause some appreciably greater trim effect at flying speeds. That probably results in the need to further trim with TX settings. And then what do you have....
BTW, did the airplane require any roll trim before and after the trim tab?
whatever............