I have some 5mm Dux here right now and I was considering the same thing. I am going to have to checkout my blocks at the top of my mast to see what kind of diameter I have. I have been using Dynex 5mm with no problems. The Dux will "kink" on every strand that is in compession. You cannot over bend this stuff. It looks like rope, but it is another annimal.
Also how would you be attaching your sails to the halyard. I am having very good luck with "softies" that are attached to the dynex soft eye halyard.
http://www.hampidjan.is/media/pdf/Dy...intainance.pdf