Notes on my installation of my Thinkpad P50, which may be rolled into the main Thinkpad P50 Arch wiki page once I confirm them as best practice.
1. in UEFI setup ("BIOS" setup), accessed via F1 at power on prior to boot, under display settings I selected Discrete (not hybrid, which was the default) to ensure that the Nvidia card would be the only display in use. This is higher power consumption but simplified use of external displays for me.
2. I installed the Nvidia driver, not nouveau. Additionally I used libva-vdpau-driver (see Hardware video acceleration.
3. I used libinput when install xorg