Best way to check luff is to hook the top or bottom to something and then pull to tighten. Don on a sail loft floor you can measure exactly.
If the boat came with only a three to one the sail wil probably not have a lot of strengthening. Check blot rope for sail stretch and maybe lenghten it. I have jut added an 8:1 to an original 4:1 to great effect but I have taken care to strengthen sail.
Cheers