I experienced much the same thing, but on less racy equipment. For years I had a 145 liter free ride board and a 9.5 cambered sail. There were about five or six days each year when I would go out on a nice low wind day but got completely overpowered when the wind came up suddenly--down in the water, hoping the wind would die overpowered. I also had a 120 liter freeride board that would carry an 8.5. I started noticing the 120 and 8.5 planed just as early as the 145 and 9.5 combination, maybe even earlier (because I could pump it on a plane earlier)--and I could handle the smaller combination better when the wind came up suddenly. The sails were the same make and same brand, so it was not due to differences in sail quality. The thing I concluded is that, for my 155 pounds (70 kg), putting up a bigger sail does not help. I think there is a point of diminishing returns and I had gone past it with the 9.5. And since I didn't need a board that would carry a 9.5, why keep it? It made my windsurfing life a lot easier.