I have been meaning to do the synthetic stays also. I would use 4mm ultrex with a breaking strength of 3150lbs for the forestay and 1/8 for the sidestays.
I was sailing my old HT with my previous co-owner after the RTI in June and we had just gotten off the beach (the fad happened to be there also) and were getting into the groove when he sheeted in the main and BLAMMM... the foresay parted. (fad did not even offer to help, but then again, a power boat graciously towed us back to shore). This was more-or-less brand new 1/8 7x19 wire (why 7x19 you ask? why not?) which we put on when we replaced all the standing rigging when we got the boat. Thankfully it broke when we were near shore, rather than the day before when he was in the Gulf.
It didn't break at the swages- it broke in the middle. He then put the original back on and used it for a few sails then realized some of the strands on that were breaking!
I made a forestay with closed thimbles for him out of 5/32 ultrex and it has worked so far (not a lot of use) but it is difficult to tension correctly because of the creep. But it is easy on the HT to adjust for this with lashings to the bow tangs for the bridle wires.
From that experience I now understand how much tension the forestay is put under. Clearly the highest loaded line of all the standing rigging - esp with max sheet tension and 400lbs on the wire near the back of the boat.