I don't believe that Oly's comments have any basis in reality. The block isn't a heat sink, the water is. The cylinder wall is just a thermal interface to conduct the heat but contain the combustion and provide for structural support. To put this into simple terms, the cylinder wall is an insulator (it really isn't that great of a thermal conductor compared to Cu, Al, or Ag). Think of your house. Are you going to keep warmer with thicker insulation or thinner insulation. I think that this really has more to do with cylinder ballooning.
Also, think about this with sleeving: There is a sleeve/block thermal interface which could include an air gap. Remember that the machining isn't perfectly smooth or round. You could be worse than what you started out with.
