I have the opportunity to purchase a left sided power meter for a decent price, that is a newer FC-R8000 arm, whereas my bike currently has Ultegra FC-6600 equipped. My assumption is that because it is only left side it should work, but I would like to gather secondary opinions before I bite the bullet. Thanks!
Compatibility between Hollowtech II crank arms is a sure thing if you are fitting road to road (not forgetting the spacer for triple) or mountain to mountain. Therefore you'll be fine with a new Ultegra on an old Ultegra. Minor details can change, Q-factor by a very small amount for instance, but it is unlikely to be detectable to you the rider and won't have an effect on frame clearances.