Not just Vne, the 15m polar at high speed is much better than the Open ships.

Are you shure about that? The modern Open class ships has the same wing load or even a little higher than most of the competitive 15m gliders. If you then add up that a high (Open) aspect ratio of the wing climbs better than a lower (15m) at the same wing load give us that an Open class ship will climb better and glide equally.