Most of the power differences between cores these days isn't in terms of clock speeds but in terms of extra structures that cause more instruction to be executed each cycle. The rule of thumb involving voltage/speed scaling is that your power use is indeed the square of your performance more or less when you're in a reasonable region. However, a relatively simple in-order core like an ARM A7 might also only take 1/4 the energy to execute a given instruction of a complex out-or-order core like an ARM A15 even when both are clocked at 1 GHz on the same process.