The thing that got the Anarchy crowd going was whether or not you could exceed windspeed downwind with such an arrangement.
The wind speed is not a barrier, either with sails or windmills.
Both power systems take energy from the wind so that the boat can use it to overcome drag and increase speed. There is no theoretical limit to the quantity of energy that can be drawn, so there is no theoretical limit to speed.
The sail / windmill sizes are limited by engineering problems mostly associated to the weight increment vis-a-vis energy and drag increments. This should cap the speed for a given boat size. But it is only an engineering problem. As sails/windmills are built lighter, speeds will increase.
