Crank Arm Compatibility - Shimano Ultegra 6800 and R8000
Yes

The website for the Stages Power Meter mounted to a Shimano Dura-Ace R9100 non-drive-side crank arm says:


  Crank Compatibility: All Shimano Hollowtech II road cranks


This also agrees with Rider_X’s comment and my own guess that they should all be compatible.