Another thing to consider:
If you have fans pushing out more CFMs than fans pulling in - that excess air has to be pulled into the case from somewhere. This will be pulled through cd-roms, floppy drives, or anything else that allows air to come through. A side effect will be dust buildup inside these devices. I believe that a slight overpressurization (is that a word?) should exist - meaning more air in than out.
-MC