I don't think that increasing the downhaul tensioning ability for that sail is really necessary. The original Hobie 18 dacron main is pretty flat to begin with, and over-rotating the mast will do the rest. If I remember right Ken Marshack experimented with tensioned diamond wires/raked spreaders and downhauling a long time ago and abandoned it. I believe that the Cat Sailing 90's book is refering mostly to more modern cut sails that start with a lot of draft that you can remove as needed with the downhaul.

John