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