I bought a couple of those recently to use with my 1070s...have a Spotswood frame on the way after finding a tower case inadequate for keeping even two GPUs cool, let alone four or more:

http://amzn.to/2sF7wm5

One didn't work at all.  The other appears to run OK at first, but as soon as the miner software starts hammering the card, the card falls off the bus and quits working until at least a reset (or did it need a power cycle?).  I went into the BIOS settings and set the PCIe slots to their slowest setting; that didn't help.

I might still have some ribbon risers hiding in a box; I never had trouble with them in the past (last used them with two Radeons (HD 6870 and HD 7750) on an Intel D945GNT), but I don't know if they'd have enough reach to load up the frame with 8 GPUs.  Is there a longer ribbon riser available than 12"?  If not, what USB (or other cable type) risers have been less troublesome than others?

(In the meantime, swapping GPUs around so the hotter-running one is on the bottom has helped.  At 125W each and fans on automatic, the upper GPU with better cooling stays in the low 70s, while the lower GPU stays in the upper 70s while mining Equihash.  Fans at 75% keep both GPUs in the 60s.)