Just ordered a new sail from Jotz Sails. It's complicated, this guy is a world champion in the El Toro Class, and he knows his sails (I would guess). On the order form, he wants the mast deflection measured, so apparently he customizes the sail to how stiff your mast is.
He says his sails work best with 1.25" of deflection and our old wooden spar has 2.25". I would anticipate a fuller leach? Maybe. I don't know. What I do know, is that 50 pounds of slate will certainly bend this mast:
Any scientists out there want to tell me how much wind correlates to 50 lbs of pressure on the middle of the mast?