So it planes early but seems to max out at a certain speed?
My first thought would be, "is the daggerboard up?" because if the daggerboard was down that would definitely keep you from doing true, full-speed planing.
Second thought is maybe the mast attachment position is too far forward and/or the board's rocker is not curved right to allow the front of the board to rise up clear of the water. Kind of ironic that your slower-to-plane windsups have higher top end, but it would make sense if their curved rockers allow them to ride on the just the tail section once way way powered up.
Third thought is maybe it's something about the little weed fin, like it's not giving you enough lift.