Cooling an MCU to boost performance
There is a very significant difference in how MCUs are designed and how desktop CPUs are designed.  Desktop CPUs are what is known as 'thermal limited' - their performance is limited by how much heat is produced.  If you keep the die temperature constant while cranking up the voltage and clock frequency, it is possible to get significantly more performance out of the chip as there is still some timing margin available.  

However, low power MCUs are a different beast.  They are generally designed for low power operation.  The circuits inside are built with larger transistors and wires, and they are tuned to operate up to the design frequency while consuming as little power as possible by trading timing margin for power consumption.  Because of this, there is very little advantage to active cooling for an MCU.  It may be possible to get a small increase if you crank up the voltage, but I doubt you will be able to get anywhere near a 2x overclock without major stability issues.