In order to have the cooling air flow or move through the cowling, you must have a pressure differential. Higher pressure where the air enters the cowling, and lower pressure where it exits the cowling. In some cases, this can be done with different sized inlet and outlet holes on the cowling... a small inlet and larger outlet. BUT.... depending on where that outlet is located on the airplane, and the angle of the surface that it is cut out from, you can have very little or no pressure difference and thus poor flow of cooling air. I have even seen reverse flow thru the cowl if the cooling air exit is located poorly. In some cases, making the exit hole bigger and bigger can make the problem worse.
In the case of the bottom of the Mustang cowl, the bottom surface is angled so that a hole cut in the bottom will actually scoop some of the air passing over it into the cowl, making high pressure in the area where you are trying to extract the cooling air. This applies to other Mustang kits such as the Aeroworks too. The solution isn't a bigger hole, it is a spoiler or cowl flap located at the front edge of the opening. This will create a low pressure area and allow the air to flow through the cowling.
KennyMac