I'm a little rusty on tyre physics, but I think you're saying that the fall-off in grip as the slip angle goes over the threshold is not enough, or doesn't come quickly enough, or both?
I appreciate that there are other dimensions that will complicate the picture, but if we graphed slip angle to grip, you're saying the curve would be wrong at the higher end of slip... but what shape is it and what should it be?
Does that mean we're also getting a little more turning than we should out of riding the edge of an understeer?