Yours is an interesting question but the execution of the experiment I'm afraid is flawed. The 20 degree difference in temperature you measured was the result of radiation plus ambient temperature. To make the experiment valid you would need to place the thermometer approximately 3" from the condenser coil half way up with the outside fan running with the condenser shielded from the solar source. Compare that result with the condenser unshielded but the thermometer shielded from the solar source. This will give you an accurate temperature differential.
That being said, there is an efficiency increase with cooler air passing through the condenser coil and it can be measured by placing an amprobe (current sensing device) across the run terminal wire of the compressor. A decrease in ambient temperature will cause a decrease in the amperage draw of the compressor thereby increasing its measurable efficiency ( assuming all other factors remain equal) I hope this gives you some more food for thought.
Of course you will need to factor in your expenditure compared with the cost saving which can only be measured with knowing the decrease in power usage times the run hours of the compressor times the cost per watt.
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