I have never heard of depth having anything to do with that. Coming from a professional car-stereo-enclosure-for-killer-bass point of view, I do know that you should have at least 2 inches from the magnet and the back of the cabinet in order to not overheat the voice coil. (yet that little vent on the back of the speaker really does something).

I know that in a bass enclosure, the size of the enclosure matters for tuning reasons, but I don't believe it is very critical in a full range speaker.