So the question for me now is; Should I get the A7R as a gap filler? Edward; Are you sufficiently happy with yours to recommend that?
No, simple answer, I've adapted to the a7r primarily because of the sensor & small footprint for Underwater use.
As a replacement for the 1Dx not in the same league, may never be (for wildlife, anything that moves quick, weather proofing, ruggedness).
As a replacement for the 5DMK III ? In most situations I believe it can & does, with the exception of Lens Support at this point. Currently I use my Canon/Zeiss Lenses with the Metabones adaptor, not a perfect situation, but it does work well.
I'll be interested to see what Sony does with the a7r II, or, wether or not Sony drop that +50MP CMOS Sensor into a DSLR, that would almost certainly see me buying into the system and if they set the Camera up to use any Lens (via adaptors), maybe getting close to what I'de like to see in a high MP Canon Body.
I think the current a7r is a good Camera with future potential as a system, it's not yet a very good camera just yet.
The 5DMK III for it's few idiosyncrasies is still a very good Camera with all the support that Canon provides, until Sony can match that it's probably not effective to look at the a7r other than as an experiment, or for a specific reason (Housed size for underwater use).
I've looked at the Fuji XT-1 for a few reasons, but a friend of mine has one and the EVF on this Camera is where I'de like to see Canon move towards in a 5DMK IV with 40MP sensor, perhaps the best iteration of an EVF I've seen, still has lag, but it's so minimal you hardly notice it, maybe where the a7r will be next year.
Of all the systems I've looked at in the last 12 to 18 months, the Pentax 645z is the best development I've seen, if they had a better Lens line up with Leaf Shutter Lenses as an option (current lens lineup sync speed is 1/200th ?? from memory), for anything other than Wildlife I'de be on board.