As I said, there is only so much power a given prop setup can transmit to the water. If you set the propt up to run at hull speed +10% then an extra 50% power simply can't be transmitted to the water by the prop. Actually, I can't think of many sailboats that are overpowered by design. Builders don't want to spend the money that larger engines represent. Nor do they want to use the extra space.