> "Is not capable of more than 55 knots calibrated airspeed at full power in level flight."

Law is a funny thing. "Level flight" is used because if you go down then obviously you can go faster and it is assumed that when ascending you'd be slower.

If you have software limiter that kicks in only when the flight is level then you can ascend 100mph and descend at 100mph and still be incapable of more than 55 knots in level flight. So legal.