perhaps look at the power losses from increasing temperature.. and then extrapolate back to work out the gain for lower temperature.
you would also need to measure the temperature of the air vs the temp of the IC, to see how much extra heat is actually extracted.

some random sites say around 1% power for every 6 degrees celcius....

Through the science of aerodynamics and cryogenics Design Engineering has developed a system to harness the cryogenic properties of liquid CO2 to lower the intake charge temperature by up to 50°F.
so that's... 5% power increase?