I personally dont think this isefficient.
I am just getting drafts for my own custom case design that will:
* Carry 2 motherboards, 4 PSU, one behind the other.
* Be 5 Rack units high.
* Be totally optimized for airflow
* Carry 5 (!) cards per motherboards on extension cables.
The goal is a 40 double wide cards per rack design
Setting up some mining operation at the moment - I order teh stuff half rack wise (40 cards) and I dont want to spend too much for rent.