I suspect there is a practical reason for the mainsails outline, like batten stiffness and length or something, but perhaps Marcus reads this and responds.

Theoretically an elliptical shape is superior to a square shape, but in practical sailing there doesn't seem to be much difference. There was a more pronounced trend towards an elliptical top in the Tornado class three years ago, but everybody are back to sqare tops now. The Tornado does taper it's mainsail towards the top a bit more tough..