[geo]

I think that in planing conditions cutouts determine the planform shape of wetted surface, rather than its extension. In fact extesion (surface) is the much that is needed to support total weight (body + board + rig etc.) at a given speed.

Otherways, it would be possible to reduce wetted surface even to zero, by designing cutouts corresponding... to the wetted surface itself.

Of course, the shape of wetted surface (together with rocker line) will affect angle of attack and therefore lift coefficient and, consequently, wetted surface extension. But to say that cutouts outrightly reduce wetted surface seems to me inappropriate at the least.