You are right the freeboard was increased by 100 mm. I have a old mark 1 which I rebuilt and modified 20 odd years ago. I cannot help you much on the mark 3 as the ones I saw were all different. I remember seeing my first mk 3 and thinking what a lovely roomy cabin the drop keel version had.
Being a keen racer then I felt the extra free board was an unnecessary drawback when going to windward in strong winds.