Im not sure, but i think it has more to do with how much air gets moved throught the radiator, so i dont think it would matter that much if you had a little gap, although you should be concerned with the area size, if airflow is only through 40% of the area, then your only cooling that much, if you made a shroud that covered the whole thing that would improve cooling, but im sure others know way more than me, they will help more