It's all about temps. The GPU can take a decent amount of voltage and the reference PCB is fairly stout in that department. Again it's all about temps, if the cooler is reference then 800 is usually the limit-ish.
Try to crank the fan up and I would not let it sustain temps over 90C. Might be noisy but stock cooling on these was just not up to the job at 800+MHz. The problem is these cards really fly with voltage bumps and a good OC. Mine has been at 962MHz @1.168 Volts for over 2 years with 0 problems, but it is liquid cooled. On air 800 is a good goal with minor voltage bump from afterburner preferably. Stone Giant DX11 and Heaven 3.0 simulate the best temps under gaming conditions from my exp. Furmark and such are overkill I would stay away from those on air as the generate unnecessary heat you wont see during gaming.