Power consumption of processor: Cubic in MIPS or linear in CPU utilisation?
One of those is an apple and the other an orange.

Systems with DVFS are capable of varying their performance in MIPS, yes. However, the frequency scaling is not exogenous but driven by the system itself in response to various factors, usually including utilization and temperature.

Utilisation varies between 0 and 1. The second formula applies whether or not you have DVFS on, but both Pmax and Pmin vary slightly according to the first formula.

So in order to really use both you need to understand how the system varies frequency according to utilisation, which may be difficult to find out.

(Other facts may complicate the picture such as ARM big/little or other systems for turning on and off whole cores in response to load.)