Did an event today, and after I got the essential shots with the 1dX, I played around with the A7R. Setup was A7R, 50mm f1.2L, 580EXII with ring flash. After a bit of faffing around, my hit rate was almost as high as the 1dX. Used the 50mm in manual mode, but used focus peaking to manually focus rather than the focus magnifier. Focus peaking is accurate and my hit rate was over 80%. The misses were usually when there were no real defined lines for focus peaking to latch on to. I found the subject's eye worked pretty good for focus peaking.
Yellow worked better than red to define the area of focus (less distracting).
580EXII worked well in manual mode. Obviously doesn't work in ETTL mode, but I generally use manual mode most of the time anyway.
Not as quick/accurate or convenient as the 1dX, but can be used if needed.
Found another 2 cons of the A7R though. Shutter lag is minor, but becomes a pain in events/ wedding photography. Not something that worried me with landscapes. Also on a tripod there can be a bit of camera flex between the body and the grip with a heavy lens (24mmTSE or 15mm Zeiss). The 1dx on the other hand is a rock. If only they had the Sony sensor in a 1d body at the A7R price.