I'm using the Asus Z270-P https://www.asus.com/us/Motherboards/PRIME-Z270-P/
Works well.

All I needed to do was update BIOS to get the "above 4g encoding" option, and enabled it!
PCIe gen is set at auto.
Running 6 GPUs without problems with smOS / simplemining.net.

Happy mining

Have gotten 7 GPUs working with the ASUS Z270-A PRIME.

Here's what I found:

-- Had to flash the BIOS.  Board shipped with 5.0.0 and I updated it to the latest (7.0.0 I think)-- this is necessary to get the "Above 4g decoding" option. Which appears on the boot menu in the bios.
-- PCIe switched to Gen2
-- Didn't make the other changes.

WARNING WARNING WARNING

When I flashed the BIOS the onboard network hardware stopped working with linux. this caused a bunch of problems.

I had to run this utility to refresh the network drivers ROM to make things work:
https://downloadcenter.intel.com/download/19186

Current setup for this board-
-- Ubuntu 16.04 server
-- 7x NVIDIA 1070 8GB Cards

Working fine. (And about %10 more hash under linux than under Windows.)