The difference is the stock fan shroud is not an inch away from the back of the radiator. And I know people will dismiss what I'm about to say but at road speeds the air will be stagnant in the areas where the aluminum shroud is parallel to the rad core.
Would you rather spray a stream of water into a funnel or at a flat.
At the flat the water (or air) has to turn 90 deg on impact, much will just bounce back. Like dropping a ball on the ground. It goes back up not sideways.
A good shroud will have a funnel shape of some type and be spaced farther away from the core, also if it is flat like your is it will have rubber flappers that open at speed and close at idle.
I'm out!
