Sailmaking kits vs. building my own.
Designing sails has evolved into a very sohisticated art and science. The best lofts have this science down pat to the point that they are using fabrics woven with specific load biases and with careful stress mapping. Careful orientation of the fabric (within 5 to 10 degrees) can make a huge difference in the strength, stretch minization, speed and durability achieved in a particular sail. I do not believe that the kits offer that level of sophiscation that is offered by most major sail lofts.
I would think that perhaps the best way to learn sail making is to take a job in a major sail loft. Today most designs are done by sophisticated computer programs. If you worked in a loft they would probably give you off hour access to the program and a decreased cost to purchase the sail panels.
It should be noted that the better lofts tend to do better fabric testing to make sure that they are getting what they ordered. They routinely reject fabric as unacceptable. That fabric goes to lesser quality lofts (including as I understand it, some kit lofts)and lofts with less buying power. The bigger company''s no longer cut sails at the local lofts but have the panels machine cut and marked at a central cumputer run facility. The sails are then assembled at the local loft.
The difference between a well designed aand poorly designed sail or one with poorer fabrics (even if they come from the same fabric manufactuers) is very significant and can really shorten the useful life of the sail, reduce its performance and result in greater heeling and less control. I woould discourage you from trying to design and make your own sails but I am also not a very big fan of the kit sails either.