The cFast, IMO, is more about video. 4K @ 60 will need the speed.
I was going to question that, but I'd forgotten it's using MJPEG for 4K (not H.264 or 265). 1DC 4K MJPEG rates seem to be around 50-70MB/s (for 24/25fps), so I guess that at 60fps on a 1Dx II you really are going to be pushing your luck with a CF card.
If the 1Dx II 60fps data rate does end up being twice that of the 1Dc (possible, as there's no inter-frame compression with MJPEG), you'll get through 1GB of storage in 7-10 seconds!