I just got rid of my old dacron on my N5.7, an went to a mylar. I had the same problem with the sails chocking in the heavies. With the main bellying out so far, it chocked off the slot. I couldent point at all. on a reach, in even moderate wind i would be over powered. It was like riding a bronco. New Mylar squaretop = new boat. The sail can be shaped any way you want it. I tightened up the diamonds, added rotation, no more chocked off slot. tons of useable and controllable power.
I think your sails are tired. Look around in the winter, there are some great deals out there.