Hello Sanyi

I am an A Cat sailer, but I think our rigs have a few things in common. If you are looking for power the sail should not be flat on top, that should only happen when the cunningham is pulled hard and the main sheet is tight. I sounds as if you could have too much prepend in the mast which is causing the sail to be too flat. I would start with straightning the mast a little. Maybe the maufacturer of the main sail can tell you how much luff curve the sail has and how much prebend is recommended for it. 15 knots and both hulls still wet is no fun. Here is an article by Andrew Landenberger that might help you too http://www.landenberger-sailing.com/spip.php?article22
Regards

Daniel