Cal 33'''' running rig
There are several aspects of your post that I want to touch on, To begin with, wire generally has lower stretch than the rope that it is paired with. Because only a short length of the rope is exposed to load, the amount of stretch in the rope becomes less critical so the rope is generally sized one or two sizes smaller when used as a tail on a wire rope halyard. Stretch is generally less of a problem with spinacker halyards as well. But mainsail and jib halyards stretch is important to the performance of the boat.
I visited the site that you referenced. I would suggest that you consider using their Tech II polyester/tecnora blend in a 3/8 diameter. One minor point, if you are using a jib in a headfoil then the stresses are a little lower and you might be able to get by with 1/2" polyester double braid. I also like to add a sacrificial cover over the line at the masthead sheave when stowed and raised and at the halyard lock or cleat. This greatlky increases the life of the line.