standing rigging diameter?

[Brion Toss]

Hello again,
First, the data I have for your boat shows a waterline of 32ft. Even at 33ft, you would probably be well shy of 45,000lbs rm at 30 degrees. A direct incline test could confirm this, and I recommend one. Meanwhile, 5/16" uppers and 9/32" lowers look right. For more details, see the "Rigger's Apprentice," or be in touch with the shop to set up a consultation.
As far as runners go, as I said before, those aft-led intermediates are not making a useful contribution, and runners will. It would not be adding needless complexity, but restoring needed complexity. And there are ways to make them very simple to use.
True, many cutter-rigged boats have sailed below the Capes without them, but hey, some of those boats didn't have standing backstays either. Your intermediates were a misguided attempt, long-since abandoned by people like Bob Perry, to get rid of runners. The attempt resulted in poor staying, compromised pointing in storms, chafed mains'ls, and a higher incidence of unintentional jibes.
As for wire strength, there are international standards for a given diameter, construction, and alloy. Whose numbers are you looking at?
Fair leads,
Brion Toss