There's nothing wrong with making the panels thicker except it makes the cabinet heavier. It's really more important that they be well braced. As long as you brace them well, you don't need the extra thickness but it won't hurt anything either.As for woofer loading and cabinet size, the four Pi provides good cone control with smooth rolloff down to about 30Hz. It won't really be completely unloaded until fl, which is under 20Hz. The response curve is slightly overdamped, so rolloff is smooth and gradual, and there's useful output down to about 30Hz.