ECAM,
The difference in climb performance/best rate speeds was determined by actual test results. I suspect that as I know we increased the drag of the aircraft, and the drag increases as a square of the speed, the faster I fly it, the more more drag the aircraft has, and the performance is thus reduced accordingly. A climb speed of less than 80 KIAS did not result in any greater improvement, and results in too little a margin to the assigned minimum maneuvering speed for the modified aircraft.
The calculation of the drag is possible, but exceeds my skills. My flight test analyst does this for me - I just fly!