Because I am work I have plenty of time to ponder this question.
Logically I dont see how this can happen for a static sail (i.e. a sail rigged but no wind or rider force exerted.
Mathgeek that I am, I calculate that the largest change in rigging force from changing the clew position is 0.2Kgs. I assume 10kg outhaul tension and that the clew position changes by 17cm. These changes are deliberately large to magnify any result.
www.seabreeze.com.au/gallery/gallery.asp?imageid=2888So if the effect is not a "static effect" then it must be a result of the way the boom is loaded up when sailing. I think it is a similar effect to raising the boomclamp height. When you raise the boom at the mast you only get a fraction of the incease in height at the harness lines (proportional to the ratio of the distance between the lines and the mast and the lines and the clew.
By raising the clew height as well as raising the clamp height you get more height increase at he harness lines, resulting in more of the riders weight being carried vertically by the boom (resulting in greater downward pressure on the mastfoot (resulting in earlier planing)) ;-)
I think this is what is going on, (but then I am better at my maths then my windsurfing).
JB