very nice! i love that 4-5U expansion case! but 3600+ $ US is so expensive.
a motherboard I got but wasn't aware of a detail which makes my solution not viable for now, is the msi krait gaming 3x z170a board. it's got 3 x16/x8 slots with 4 x1. the problem at the moment are the skylake cpus, they only have 16 pcie lanes. so hopefully intel releases better skylake processor. All that to say only 4 RX 480 works on my current board, when i add a 5th i get an error message stating there isn't enough free resources code 12.
otherwise i would suggest the Asus X99-E WS/USB 3.1 , it's got 7 x16/x8 slots. I'm looking at this one, because i got some Xeon processors at home.
and i use pcie risers with usb3 cables
The 7 PCIE slot motherboard are the high end ones. They are much more expensive. Not worth the mining.
I have one rig that has 7 x R7-370s, using standard H81 BTC Pro mobo and 3-to-1 PCI multi-plex card.
It started as an experiment but I ended up keeping it as a one of my test systems for EthOS and also Claymore.
R7-370 runs only about 14.5Mhs per card and its not a fast card, however its very cool and not power hungry. So this is good card for this rig. In order to use higher end cards for 7/8 GPU rig, prepare to tinker on multi PSU setups.
In conclusion, I will never do multi GPU rig like this again because, it requires expensive 1600w PSU, larger than usual open cage rig, plenty of USB risers and many hours of tickering, only Linux/Ubuntu can drive more than 8 x GPUs - higher learning curve because you need to install AMD drivers separately in Ubuntu 16.04, for Windows... I think limited to 7-8 or more GPUs and requires special hacks.
To consider a 8 or more x PCI slot mobo and PCI expansion sub-board; similar to deep learning multi GPU computers... for mining, its not worth it - it will be too expensive.