Sad it did not work out for you. No range war intended just some feedback.
I have some issues with the GFX100, but so far it's images seem to surpass what I was getting with the 50s, Different stokes different folks I guess.
1. Sharpness, I agree 50s out of the gate images seem sharper, but in C1 with default sharpening settings, I see no issues expect on my copy of the 23mm, which always has been just a bit soft for me. What slight degree of softness I get can easily be regained in post (at least for my older eyes).
2. DMAX, I guess I expected something better, it's a new chip, with totally new design, and the same chip on the IQ4 greatly supasses the IQ3. What I see is pretty much the same DMAX from ISO50 to ISO 400 which surprised me, and about a 2.5 stop push max. Past that and noise will start to become an issue. Much past ISO 1600, and images become considerably less productive.
3. IBIS makes a difference for me, as hand holding the 250 and 250 with TC and the 100-200 produces excellent results for me.
4. Banding, I just don't see, but I never saw it on the 50s either. I realize if you push a GFX100 file enough, it will band in the shadows, plenty written about that, but within my range 1.5 to 2.5 stops of push I just don't see it.
Issues I have that bother me.
AF, is just as bad as the 50s, in that with any subject shot in low light or low contrast, you will odds are will either get a total miss or an image slightly out of focus. It's surprising to me just how fine the line is, for at times I can hit a distant ridge line where I have plenty of contrast and still miss AF. I saw with with the 50s also and as I stated I just don't any improvement. The Z7 hits all the time every time, amazing AF in low light, low contrast compared to the GFX100.
Low light images will tend to take on a red cast, so early sunrise and late evening shot are a bit tricky. This carries over to the raw, as it's not just the LCD white balance. As soon as normal light becomes available, WB is excellent. So far I have been able to easily adjust in post
Write speed, is slow and will buffer at times especially when shooting in exposure bracketing mode. Even with a Lexar 2000x card the camera will bog down at times. Not as often with the 2000x card but XQD or similar card support would have been a nice feature which I am sure will come with the next camera.
I only use C1 for my GFX100 images, as I find LR pulls much more noise by default. C1 20 seems to do an overall excellent job.
Paul C