The main thing it boil's down to is STABILITY! Like someone else already stated, the downtime is your biggest loss. Also who wants the headaches of that many GPU's on one rig. Because i would be so paranoid everytime the fan speed changed thinking. " is it gonna crash" .
Just stick with 6 GPU's or maybe 7 per rig.
BTW, i tried those PCIE expansion cards, DID not work for me and my MB. But they do work just not for every MB. IMO those are only good for if you only have like 2 or 3 PCIE slots period on a MB. Then you could expand to 6 PCIE on a MB with only 2 or 3 PCIE slots, IF the MB and CPU like to play well together.