I can confirm that the board works with 6 gpus without problems. Just enable above 4g decoding and put your PCI latency timer to 96, 16 for every card.
If you enable above 4g decoding after you installed windows or another OS you have to reinstall it. So first set your bios settings, than install your OS.
My only problem with the board is that I only get a monitor signal when I’m in windows and not before. If I unplug all gpus and plug it into the onboard graphics it works and I can edit the bios or install an OS.
last but not least, you need a kaby lake CPU for more than 4 gpus on z270, so Skylake won’t work. I use the Pentium G4400.