whether you've had 6 subarus or just 3 like me....

The pocket in the front of the original bumper trapped the air, it did not vent it.

Now, I am no aerodynamicist, but I think that unless there was some sort of vortex created by the old air dam (which I do not believe to be the basis of your opinion/argument), the air will pass with less abrasion around the new bumper.