Since your unit is filled with R410, the high pressure needed to dump heat on a hot day takes a lot more power. Research has proven that above 110 degF condenser coil temperature, R410-based systems are actually less efficient than the older systems running with R22 due to the enormous pressure needed to force condensation. So on those super hot days, cooling the intake air would be highly effective. Likewise, on frigid cold nights, preheating the intake air would be effective also.
The unknown here is this: at what temperature does it take more power to pump water than the heat that water will transfer? The answer: you'll have to build it and run experiments. There will be a balance point during those somewhat hot or cold days where you would be better off letting the unit do all its own work. Or you could spray water on the coils directly and gain more heat transfer. Again, no way for me to tell until you do it.
Something to consider about directly spraying the outdoor unit: a little water goes a long way. You don't have to flood or drown your coils to move lots of heat. An awesome example of this statement is the Coolerado. It uses a thermodynamic cycle called the Maisotsenko Cycle to do the same thing you hope to accomplish with your radiator. Their units consume 2 gallons or less of water per hour per ton of cooling.
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