Can’t it be overclocked without increasing the voltage? I mean, increasing the voltage asks for more electrons charging or discharging the parasitic gate capacitances of the mosfet switches, so that alone increases already the power dissipation without forcibly speeding up things, while increasing the clock frequency would require faster switching times, which would, at least in theoretical physics, ask for smaller charges, rather obtainable by lowering the threshold voltages through lowering the supply voltage.
But as I said, that’s pure theory, just thinking out loud...