I think you're right in that there hasn't been a huge leap in sensor performance since the original A7S...although it is rumoured to be followed up later this year.
The problem (for some) with the Sony and Nikon cameras are the spatial filtering they seem to employ in their firmware to smooth out their raw files. Canon doesn't seem to do the same dirty tricks. A 6D is pretty solid, but getting a bit long in the tooth.
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