In the course of building that is reported on the Building site, several test sheet lines were used; Spiderwire 80, Spectra 65 and 80. The Spiderwire was to heavy aerodynamically and with high drag through the fairleads. It also tends to fray at the collector loop. I finally settled on Spectra 80, as it is easy for tie bowline loops, as opposed to 65, and its profile is not that much greater. One does not think high test is needed here but I have found that wild gybes at the leeward mark or very stressful on the sheets and the adjusters.
Stainless seven strand .018" wire is just fine for the the shrouds to the gunwale. Good tight wire will fit into a #2 sleeve but the #3 is fine also. Some use .015, however, because I use .018 for the forestay and backstay, I have reduced my inventory by using it all the way around. Aerodynamically, it is fine.
Most shroud failures are caused by collisions or gybes where the main boom hits the aft shroud smartly. These are rare with .018. Other failures are caused by the lack of fairing the exit from the internal tang through the mast, as Robert has pointed out. This is most comon on the B rig at the aft shroud. I have converted my B rigs to external tangs to prevent failures even with careful fairing.
Rick