Have you measured this?

I would expect that overclocking actually increases the battery life in some cases. This is because executing code is faster at higher clock rate, thus the CPU spends more of it's time idling at low frequency.

Normal usage profile of cell phone is such that CPU is rarely under constant load. Probably you would see increased battery consumption under heavy load (where some process keeps CPU at higher clock rate for fixed period of time), for example playing a video. But doing the usual stuff of browsing the web, running occasional app and making phonecalls the effect could be quite the opposite.