Weird, Kepler auto adjusts the voltage up to 1.21v. That's max and from what I've read a reference card can't override this without an epower board. There was a really cool article from kingpin and the mods they did to the card.
Edit: guess what I'm getting at is there is no direct voltage control. If you up the clock and set the power level to 132%, the card will adjust voltage up to 1.21v to keep it stable. If you set the voltage manually, it is ignored.
This is all new to me too, so I could be reading this all wrong, but this was my understanding.