Wings were starting to somewhere stall. Wings are most efficient at low angles of attack even before stalling. Better lift to drag ratio.

Another likely scenario is that reduced front wing angle changed the airflow onto the rear of the car, shifting the balance rearward.

Ignoring the aero considerations, the loss of yaw damping that comes with speed should result in lower lateral accels.

A rolling tire acts like a shock absorber (velocity dependent damper) to lateral force. But the damping rate (and thus the yaw damping) is inversely proportional to the speed the tire is rolling at. So bumps and disturbances that would get damped out at lower speeds might be problematic at higher speeds. I would think a skilled driver would sense this one way or another and drive accordingly. I am not skilled enough to test my cars limits at 80MPH, so I can't contribute personal experience.