I find this an interesting point but I do not think most sailmakers will make a sail specifically for you. They hvae a template (or whatever) and they use it. I have talked to some about even making a more UNI shaped sail for the blade and they did not seem the least bit interested. That being said, not sure I could make much of any benefit it may or may not have. If some sailmakers aren't interested in the difference between a uni sail and a 2-up sail then not sure they give a crap about whether you weigh 200 or 160. You think they would care about whether you predominantly sail in 5kn vs 20kn of wind... but not sure they care about that either. Whether one could make use of the difference is another discussion.
Anyway, my thoughts on the matter and it could just be me and my limited experience with sailmakers. I have gotten 2 new mains for cats in the last 3 or so years and the first builder (very well known) didn't ask me a single thing about how much I weigh and how much wind, etc. Of course I knew nothing then though.
If you truly want one designed for you then you do need to go to your local shop or somehow convince the big guys to give a crap about a possibly one-off sail.