Cooling Airflow
Cooling air becomes heated as it passes through the radiator. It enters the radiator at ambient temperature and exits the radiator at some increased temperature. It is the difference between the average core, or coolant temperature and the average of these two cooling air temperatures that creates the ability of the radiator to transfer heat to the air. The slower the air passes through the radiator, the higher will be its exit temperature and the higher will be the average cooling air temperature. The higher the average cooling air temperature, the less heat will be transferred from the coolant to the air. On the contrary, the faster the air flows through the core, the less it will increase in temperature on its way through, making the exit temperature and the average cooling air temperature lower. This increases the differential between the average core temperature and the average air temperature, increasing the heat transfer. Increasing airflow by reducing air restrictions, will all improve heat transfer and cooling.          
Anything you can do to improve airflow through the radiator core will help. Anything that blocks or slows airflow, either before or after the radiator, will hurt.