Line for preventer?

[Brion Toss]

Hi there,
I used to use Sta-Set for the deck pieces, and Regatta Braid for the boom. The Regatta was the short run, so didn't stretch much, but it had enough give that it could absorb shock loads. The Sta-Set was too elastic, though, so I switched to Sta-Set-X. Now that that is becoming scarce, I might go to VPC or the like. Oh, and nowadays on the boom I use Spectra, but with a short piece of bungy cord inserted, for shock load. The Spectra is much lighter, doesn't soak up water, doesn't hang down so readily.
5/16" for a preventer on a 36ft boat? Way overkill.
As for the weak link question, there shouldn't be one; anything in this system that can be counted on to part in an emergency can be counted on to part when you don't want it to.
Fair leads,
Brion Toss