My honest opinion is that the proposed setup will be a condensation/hardware death nightmare.

If it were me I'd minimize the risk by only going subzero on the CPU via a single stage, which will be much easier to manage the condensation issue.

With a proper ambient watercooling solution, you should be able to do 900-1000MHz on your video cards, which in quad SLI are going to be bottlenecked by your CPU speed. (probably even at stock speeds)