For the uninitiated, a heatsink won't actually improve overclocking, quite the contrary, heatsinks used by overclockers are not used to make the device easier to overclock, but to increase the livespan of the device. A ten degrees (Celcius/Kelvin) increase in temperature is said to halve the life expectancy of the device. If you try to overclock a $400 video card, it makes sense to increase its already existing cooling capabilities, but for a PI its probably less useful, especially if you want to reach the highest possible speeds!
Yes over-volting makes a digital circuit faster, because internal parasitic capacitances will charge faster. But you can over-volt a SoC to such a degree that the extra lost energy heats it up to such a degree that the lifetime of the chip is threatened. In that case it might be important to lower the temperature so as not to damage the chip. In addition, the structures in modern SoC's are so small that too much voltage across them may actually damage a transistor by blowing up tracks or breaking down the isolation between a gate and underlaying silicon. This results in chips with many shorts in them.
The basic premise indeed is that lowering the temperature of silicon will slow its switching speed. however the effect is quite small, and may be offset by other effects, like varying the offsets when the transistor will switch. On the other hands physicist will claim that electron mobility in silicon actually decreases with temperature, the jury seems not to be out on the matter.. Although super cooling remains popular, so maybe extreme cooling hits a "sweet spot" giving you a few percentage points more speed.
In any case it doesn't matter, the main reason for a heatsink still is to keep the silicon cold enough when overvolting, so as to not reduce its lifetime too much.
more info here: http://electronics.stackexchange.com/qu ... mperatures