This is well known, and even those who accept Nokia's word as gospel, should accept that. In the words of Nokia's engineer, Igor Stoppa:

"The highest stable frequency available for the currently set voltage yields the maximum efficiency."

So overclocking is completely irrelevant, and 600MHz is an arbitrary number. You set the voltage level you want to run (lower is better), and run the CPU at the maximum frequency possible with that voltage. We know that Nokia (and TI) think it is safe to run at 1.35V.