The lower grille pan is an important part to direct air flow. Air comes in the grille, and if the lower pan is missing, air will take the path of least resistance, exiting below the radiator. Your radiator itself offers a large resistance to air flow, and with the shroud, even more. At higher speeds, the different air flow ratio is multiplied. Without further changes, take a test run with the lower pan installed.
On another subject, copper is most definitely more heat conductive than aluminum. (If money were no object, silver would cool more efficiently than would copper). The size, number, and configuration of tubes and fins being the same (which they are not), Brass tanks with copper tubes/brass fins are superior to aluminum in everything but the initial cost of materials/labor, and there's your reason aluminum is used in modern applications.