304 vs. 316

[Brion Toss]

Hi there,
We have a couple of variables to deal with. First, we do not know the original quality of the wire, nor that of the swage. And second, we do not know the state of tune. We do know the multihulls, especially hard-sailed multihulls, tend to impose shock loads on rigging by their nature, and that therefore any compromises in quality or tune will bring about wire failure more quickly than with most monohulls.
The Corsairs are great designs, but fabrication and installation vary in quality. I think it's likely that 3/8" is appropriate for your boat, and that the above variables are the likely culprits, in combination.
Some type 316 wires can be 15% or more weaker than those made from 302, and this might possibly be a contributing factor to future failures. Does the rigger have any data on average break strengths for the wire they used? It should be findable, if it is good quality. Find out that strength, and then check with the designer on shroud loads to discover what your actual safety factor is; you might find that, once you are up on quality and tune, that even a weaker 316 is plenty strong.
The tropics are extremely hard on wire, which is why we bother with 316 in the first place. So if you find that your 316 is notably weak, and that it might compromise your safety factor, I'd go with a better grade of 316 rather than 304.
As for the furler, what brand is it? It would be extremely unusual that there be no mechanism whereby you could see the wire. In any event, replace that jibstay, pronto; it was under the same loads as those failing shrouds.
Finally, do you have gear to take up slack on the leeward shroud? This is a critical piece of multihull tuning gear, as flexion of the hulls will always put a lot of slack, and thus schock loads, into the shrouds. A stout bungy cord can make an amazing difference, but you can also use block-and-tackle.
Fair leads,
Brion Toss