The other day I noticed a thread similar to this one. I didnt notice it because it is titled something about the ZL1 rear bumper rather than diffuser. They have a lot of pictures that I wasnt able to get yet and some good information about a different method of installing it.
Instead of cutting off the tabs on the diffuser, they are cutting holes in the actual bumper facia. I dont know if it will make a difference in the end product or not. Bit I didnt want to permanatly modify my bumper in case I wanted to change it some day.
Check our their thread here!
http://camaro5.com/forums/showthread.php?t=190372