The lowering of the seats definitely helps. It allows the sound to travel better. You'll also have slightly less trunk rattling. The only other way of improving how the sounds would travel into the cabin would be to remove the rear 6x9's completely. That would leave 2 open pathways for sounds to travel through.

As for the XLC's, I too have seem them in action before and they can put up #'s. But, like you mentioned, there were times when there were more than 2 subs. Also, the enclosures that I saw these subs in were huge and mostly likely ported. Another thing, maybe XLC's don't carry very well outside and away from the car... I know for a fact that the 15L7's do....