Drilling holes in the bumper cover has come up before on here. The consensus seems to be that the holes still increase turbulence. You should strive to make things smooth and tapered like a dolphin or a seal's shape. Deezler has built a nice rear diffuser which should reduce lift and reduce drag at the same time.
http://ecomodder.com/forum/showthrea...ad-7868-4.html
If you are going to cut up a bumper cover, this would be a better way to go.