At any given driving velocity your dynamic pressure is fixed.The only way to get more cooling is to streamline the ductwork leading to the radiator.
You'd need a minimized size,perfect inlet which is radiused like a velocity stack and an airtight diverging duct leading to the heat exchanger.
Without a wind tunnel,I don't think we can 'guess' our way to a better extractor unless you can duct that as well.If so,then follow good racing applications.