Pete,

DO NOT buy a new sail, for one thing, baggy/full sails do NOT work!! They are dog slow. Just ask people who have done sail development programmes!

You will suffer in the ligher wind speeds, but I would still expect you to be fully/over powered in 15kts (top F4).

As Rolf says, get people to sail behind you and look at the leach profile and how the sails are setting. you should be well powered up in 15 kts!!!

Next time youy are at the boat, take some pictures with the sail up to show us...