First off going above 2.8 (I believe) voids your warranty.
That said I have put mine up to 3.2 V and even at load the heat spreaders have barely been warm to the touch. I would not recommend putting it that high though.
I would say you can safely put it to 2.9.
With my sticks though, more voltage doesn't help. The sticks top out where they top out regardless of the voltage I give 'em. Yours might be different though.