Dennis,
May I jump in and answer, as an offshore sailor? I sail with rather less than SA/D = 15, and find my performance to be good enough. Offshore, in a cap-full of wind, I'd rather have a smaller sail, not so heavily reefed, than a large sail, so heavily reefed that it is difficult to control the twist. Downwind, I never feel short of performance, as any junk rig, of any size, is more efficient than the same size of bermudan rig (Poor unfortunate people, they use a spinnaker because they must). Upwind, it's better to have a smaller, more efficient sail than a larger, not so efficient sail; that's why I favour wing sails, and why I am tending towards higher aspect ratios.
I would say that SA/D = 15 is enough for offshore use, and the further offshore, the less camber is necessary. Of course, there are times when you would like all the drive you can get, but on average, over a long passage, a flatter, smaller sail is easier to live with than a fuller, larger sail.