I've owned the 85L for a grand 3 days now and so far, I've found it to work pretty accurately on the 5D3. I'd guess the focusing would be even better on the 1DX.
Interesting - do you use af point expansion or single point af? Just looking at the specs, a 5d3/1dx center point shouldn't be better at af'ing than the 5d2/6d if the f2.8 line picks up, but on the other hand everybody states that f1.2 on 5d2/6d is horrible to use and that's why I've given up on the idea to buy such a fast lens... but mostly because focus & recompose from the center point is so tricky.
I use both AF point expansion as well as single point AF. I've taken a fair few shots of my daughters with the corner AF points with a pretty good hit rate. For the 5D3, the 85L falls in group-A where you have the additional 20 cross type AF points (f/4 vertical and f/5.6 horizontal) enabled (see blue focus points in below image).
The images which were out of focus were more because of my poor technique than the gear. I've found rocking back or forward ever so slightly when shooting with very limited DOF will render the shot OOF.
Same goes for situations where the subject is in motion. I had a hell of a time last night trying to shoot my 1 year old niece in very low light - she was seldom still and I got plenty of OOF shots shooting wide open - stopping down to f/2 and more increasing the DOF improved the images.
As far as I am concerned, the 85L focuses excellently with the outer points on the 5D3 unless it is user error and/or subject is in motion. "Sincere" doubts over its focusing ability without having tested the combo are obviously unfounded.
I've also heard (maybe someone can confirm if I am correct) that the 85L focuses faster on the 1DX. Supposedly, the 1DX supplies additional power to the lens motor which makes it faster.