It is my understanding that spectra may be strong enough, but it will probably (a) have too much stretch to be used for shrouds, and (b) be susceptible to weakening via prolonged exposure to UV, such that it would not, in general, be recommended for this use.
The only (synthetic) rope
that I know that is used for shrouds is called PBO, but that is, as far as I know, more expensive than stainless steel
wire, let alone galvanised. Vectran is less stretchy than spectra but probably still too stretchy for shrouds (and expensive)
If you are in a real bind, kevlar might be a possibility, as it is less stretchy than spectra, but I don't know for sure and you would need to ask aqualified rigger.
Frankly, in terms of dollar for dollar value, when taking into account both up-front cost and longlevity, steel
shrouds (whether galv or stainless) is probably going to offer best value for money
(Caveat: This is my unqualified opinion, which isn't worth a whole lot
Oh, and before I forget: