Maybe you are more lucky than me but I could not detect any gpu using an M.2 to pci-e adapter. Got refunded at least.

I do have a 8 (RX480 8Gb) GPU rig running from a z270 with 1x3 pci-e switch.

But I am using about 1500w of power. I am not undervoltage alot(-96mv) with memory timing shift and a little over clock on memory. But my aim is to get zero memory errors and have a stable system.

Keep us posted on your m. 2 test run. Hope it works for you