Hope to see better numbers but it's kinda different comparing KnC hardware with HF and CT as KnC's 28nm chip with 4 cores only runs at 110-125 Gh so 4 chips are necessary to make a 550Gh unit so there is no room for expansion in the case at all, especially with the huge heat sinks that they used as well as the circuit board for each chip.  Even if they used water cooling it would be hard to imagine how they would cool just another two chips (having a total of 6 chips) and the boards attached to them and still fitting it in a 4u case.