The extra width is also needed for the curved boards since the lift forces on the curved boards decrease righting moment via the leward board.
Eh? The leeward board is pushing the hull up, thus increasing RM!!!
Yeah but since the board is also curved - some of that force directed upwards at the tip end of the board is going to result in a "torquing" of the hull, meaning that its effectively going to rotate the leward hull outboards.. increasing the amount of capsize moment.
You might be right though that the effect of the board lifting overall would equalize the effect of the boat spinning the leward hull outwards.
