There are two obvious improvements one could make to the stock M rudders:
First, use an airfoil shape rather than a flat plate. At M speeds that would mean that the widest part, 1/3 back from the leading edge, would need to be about 15% of the chord. Less balance (area forward of the stock) would be required.
Second, fill the space at the top of the rudder, going as close to the hull as practical. This will improve both efficiency and turning force.
The factory knew both of these things, and apparently decided that the improvement in performance was not worth the cost.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida