As ffar as I know, this should indeed make thhe rates better, when you light up all the caves and the surface that is. That is becuase the mob spawning algoritm goes from the loweset sub-chunck to thhe top sub-chuck looking for blocks thhat are spawnable, whhen there are no blocks above, it goes down inmeadeatly. As far as I know, aikar didn't change the mob spawning mechanics in suchh a way that that changes, I mighht be wrong though.
