I totally understand what your saying. The same frontal opening, with the back of the duct, say for example, covering the entire radiator OR half of it.
If you don't build the duct to cover the entire frontal area you are losing out on cooling capacity. You will only be cooling half the rad, and its harder to force the same amount of air through half the fins.
Evan
BTW, did anybody read this thing about ducting in Maximum boost? Basically said as long as frontal area is 25% or more then the back of the duct you are still gaining efficiency due to the air not bouncing off the front and going around the sides.