I am not going to disagree with anythig that has been posted here, but just thinking about your question did give me pause to wonder.
It had not occurred tome that people would be carrying spare shrouds (or, indeed, any spare standing rigging). It seems to me that you can carry too many spares, after all, space on a cruising boat is usually rather limited. I mean, it seems unlikely that one would bust a
shroud without having done or doing significant damage to a spreader and or the mast....ain't gonna carry a spare mast, no? I would hazard a guess that one could jury rig standing rigging using Vectran or even
Spectra, or, at a pinch, halyards. Is the extra weight in spare standing rigging really worth it?
Furthermore, on the subject of "upsizing" wire gauges. It might only be a tiny thing, but using larger gauge wire for your shrouds will have a negative impact on your righting moment. A relatively small negative impact, to be sure, but given that some relatively well known production
boats do not seem to have particularly good righting moment anyway....
Like I say, not disagreeing with the learned people here, just thinking out loud...