I'd be more of the mind that the 8 AM Speedy's VG to Roadtown isn't an hour, but that doesn't give you any time gain if there isn't an earlier ferry. Also could get hung out to dry with that 10 AM RT to CA, if it's late, you are sunk anyway. Assume a full ferry, because to do otherwise is always going to bite you back.
I guess, I'd be inclined to go with the Cape Air at 11:30 ( unless you can charter earlier ??). But then the $$ gets into air vs water taxi.
Does the Speedy's ferry from VG stop first at Trellis Bay ? Is there an earlier flight EIS to STT ?