A saggy headstay actually pulls the mast forward. Think about the geometry. If I add sag to a line, the points at the end must come closer together, resulting in the top of the mast coming forward.
I understand the geometry but disagree with the conclusion. Are you assuming the headstay is a straight line and pressure from the Genoa causes the headstay to sag and pull the mast forward?
At max headstay length the sag is built in. Grab the upper shroud at 1300 lbs and pull. Pretty tight. Now try the same thing with the loose and wobbly headstay. You can add quite a few turns before it will be tight.
