I use Afterburner, latest edition that has full support for the 680. Apparently, when the cards are under heavy load, they don't fluctuate the clocks at all, it finds a happy medium and sticks to it. Anands review
gives a very good rundown of what's going on, pretty much matches up what's going on with mine, as far as i can tell anyway. As i've said before, heat plays a crucial role, if the GPU is over 70c it simply will not hit the maximum Boost of 1110, it scales the clocks according to load, heat and power consumption, although never dropping below 1006Mhz.