Farlo commented that there must be a limit (60 Knts?) where windsurfing becomes impossible.
A good question. If WS is easiest when exceeding the true wind speed then in a decent blow sailing at less than the true wind speed the loading on sail and pilot increases and sheeting out causes the fin to ask what the *** is going on while the nose is being blown about.
Sailing close-hauled is an option depending on where you want to end up, having already changed down in sail and fin size, but on the slippery slope of the loss of speed and pointing power. In those conditions is a smaller board with inboard straps a better idea than an outboard stance on wide board?
Kite boards seem to side step such issues but there are no free lunches I guess and they have their own issues like the large amount of real estate they need with their lines.