Well I modelled 3 holes into my 1:1 scale 3d model of my car, and ran CFD on it. The holes cut through the bumper and come out in the space behind it which catches some airflow near the spare wheel. The colour contours show air velocity at the back of the car.
Forgive me if I don't rush out and grab a drill - can't see much of a difference.
If anything, the top model (with the holes) appears to have a larger area of low velocity air - hence more drag.
