CPU itself has really only some effect on the total use-time. Display eats most of the power, cellmo is eating a lot with network traffic. Having higher speeds usually allows cpu to go back to idle sooner, so, indeed, it might even prolong the battery life if you overclock the CPU. Having said that, I don't especially suggest anybody to do it. I can only say that I have seen it running at 650, so within the suggested 10%.