Rolf,
As pointed out Hobie put this type of design on their Tiger and I'm told it really works well for them. Most of the F18s are moving in this direction now.
It is F16 legal, the area for the larger head comes out of the leach producing a fairly straight leach on the sail. If you have a look at an F16 sail with a smaller head you will notice the leach has much more of a round. Make it straight and you have all this extra sail area that you can put in the head.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!