Ubuntu won't boot with more than 4 GPU's
More news.

I can get the system to boot with 6 cards on Ubuntu 14.04 server with 14.9 ati drivers.

However, it isn't stable. Hashcat runs but it seems to hang after a few minutes. At this point I'm thinking to beef up the 750w to a 1300w 80plus gold PSU. Then distribute the cards evenly across both psu's.

Does this sound like a reasonable approach for the symptoms I'm seeing? Can't see what else it could be at this point.
Quite reasonable.
In the interest of helping others, I finally figured this out last week.

The issue was running PCIE1x simulatenously with 4x and 16x. The solution was to set a kernel boot option in grub of iommo=soft.

All cards up and running at this point.

hope this helps someone else out.