Unregistered: I'll try that as soon as the water temps around here are over 5 degrees celcius, till then i'll have to speculate.
andretsin: very interesting about the concave foil's max force yet poor lift to drag. So going to a wing sail wouldn't help downwind speed in this case and you couldn't substitute a smaller wing sail in place of a larger concave sail?
A thought on ergonomics. I'm sure the lrage amount of "heeling force" that the concave foil makes at a high angle of attack is taken into account in windsurfing design and allows windsurfers to hang body weight off the rig, (counterbalancing heeling force) and water start at high angles of attack ( ie sheeted in).
Maybe a wing foil would force the rider into a very upright and on-top-of-the board position that would be very unwindsurf like? ( Though they're always talking about this kind of stance in freestyle equipment design).
It's interesting that you say a concave foil inherently generates more power than a convex/ flat foil, because in my little anecdote a stationary (no apparent wind, true wind only) 4m kite with aforementioned foil shape seemed to have much more static power than a stationary 5.0 windsurf sail. This might have been due to wind gradient factors, ect ect, but at zero degrees angle of attack it still pulled hard in 15 knots of winter wind.
It would make more sense though that this convex/flat foil kite would be optimized to fly in 80 km/hr and above airspeeds, as a kite can reach these speeds power diving even if the sailor is only traveling at 40 km/hr.