This is exactly what I was going to say. Anything that is blocked by the headrests will easily show up in the side mirrors if they are properly adjusted. I checked it out on the way in this morning to confirm, since the headrests have never been something I noticed to be an issue before.
Agreed, a non-issue after 17K miles and over 1 year of driving the CX-5 rear seats in up position. Therefore I will retain US-spec rear headrests because I do want to have my passengers protected from serious whiplash injuries.