I did a lot of testing of this theory also. I fired the sub up into the dash and tried it firing down into the floor.

I personally didn't notice much difference. Facing up, and facing the floor the only difference was some songs you could feel the sub a little better as in the vibrations. Sound quality was the same in my mind.

I had the new box designed to fire forwards more for looks then anything. Remember a sub has to move air. When its in a sealed box, it still needs to move air off the woofer cone, if the air space is to small the sub will be less efficient. Firing into the drivers seat or forward gives you the bling factor, also you get that rush of air from it, and you can starve it for air to move.

Just my thoughts.