I am sure they build in some safety factor on their tubing and what not when designing those co2 guns, however I would think they no more than doubled the max pressure of C02. Also remember that pressure tubing has a max pressure and a max working pressure. Exceeding the max working pressure will eventually work harden the steel and cause a rupture.
I would stay on the safe side and not go higher than 1250 psi. I had a QB I had converted over to HPA by Mountain Air in .25 caliber. I was running 1200 psi and getting 750fps with 21 grain diana pellets. Never had a problem with it.