Are you saying the 'stock' Mystery 5.5 sail has more sail area than the F18, square top sail? I would think any square top would project more sail area than the equivalent pin top? I know my F16 square top main has a LOT more sail area up high were you want it, than my Prindle 18 pin top. In fact, I run the Prindle pin head in big wind, just because it has so much LESS sail area up high.