The structural stability of the case provides a ~5-10GHs increase in hash-rate vs not having the rig cased.
How does that work?
I'm not sure, but I do know heavy boards like GPUs with bad heat sink designs can flex the card deteriorating the thermal contact with the GPU chip.
Either that or more directed, controlled airflow helps with temps a lot. 5-10GH over that many miners really isn't much.