On a yacht, where rig size is limited (either by exterior dimensions or via the handicap measurement) what you say is correct. C class cats, for example, have very specific limits.
On a windsurfer, however, you can use whatever rig you want, so it would follow that the best possible L/D would produce the best result. IMO, the 'gray area' of wing viability is the boundary conditions. Windsurfers have slightly more than 1:1 speed to wind speed ratio (for comparison, Yellow Pages just went over 50 kts in less than 25 kts of wind which is in excess of 2:1). So to go 50 knots on a windsurfer, you are looking at nearly 50 knots of wind speed. The impact on the water surface, gust dynamics etc is extreme, and so far it has seemed that the advantages that result from the ability of a properly designed conventional rig to 'autotune' outweigh the disadvantage of the intrinsically lower L/D.
My 2 cents, for what they are worth!