Insurance is only going to reimburse the value, which on wear items like sails and canvas is the depreciated value. SG is being generous on a 10y.o sail but in any case your $300 sail claim is now below your deductible
This whole tale is yet another reason to carefully study your insurance policy and clearly and realistically factor your risk coverage requirement. If you could find an insurer who would pay for your torn sails you would not be happy with the premium.