I have made some research on low power microcontroller techniques but was still unsatisfied with the high power consumption. As I was searching for a solution, I thought about adjusting the PLL values to get a lower clock frequency or even overclock the chip (if you really need maximum performance). And because there was no library for this, I have of course created one 
Have a look at it. The code is well commented so that you understand everything.
DuePower.zip (3.58 KB)