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I am a bit disappointed though, to me, one of the big reasons to bump up sensor sizes is DR.
DR is related to but not determined by sensor size. Fundamentally it comes down to the difference between the saturation point (full well capacity) and the noise floor. Larger pixels can store more charge, but at the same time it can increase read noise, so the net gain is less than expected.
But at base ISO for the A7rii we are looking at 13.9 according to DXO, while the ISO 64 on the d810 has a 14.8, which is already almost a stop. I think the GFX50 will be about a half stop better than the d810, or closer 1.5 stops better than the a7rii, which has real implications for some applications.
The D810 sensor is special, being the only 4th generation Exmor in production. The increased well depth over the 3rd generation (D800/A7R) is responsible for the increased DR (along with the base sensitivity of ISO 64).
The 5th generation Exmor is a BSI version of the 3rd generation pixel structure, this is why the A7RII doesn't have the DR of the D810 (but does have increased sensitivity in low light). I believe the 6th generation is supposed to have the increased well depth too so we should see a similar bump in DR when those sensors make it into a production camera.
Long story short, I wouldn't upgrade to the GFX for DR alone, as we could see a 35mm sensor with new technology this year whilst the GFX line will take longer to benefit from this (due to the slower product cycle of MF).