Then it wouldn't be stock, would it? He asked about a STOCK motor.
Along those lines, I think 10psi would probably be about it. And no guarantees it would hold that, since technically it's not really made for it. But....you asked. lol
Well, I think forged pistons and maybe some decent 340 rods wouldn't be breaking the bank too badly. And getting some kind of boost ignition retard unit and a water~methanol injection would probably allow you to run 15 psi safely.