No surprise if you ask me. I have a bunch of reservations about the GFX, but IQ isn't one of them. This beast is as ISOless as they come, dynamic range is through the roof. A 4 stop push in Lightroom from ISO 100 to 1600 virtually gives the same result as a native ISO 1600 shot of the same scene with the same shutter speed and aperture. You simply won't be able to spot any meaningful difference.
I have made ISO samples from 50 to 102400 with 4 different RAW converters (Lightroom 6.9 beta, Silkypix Pro 8, Iridient 3 and the built-in camera processor), and it's certainly short of amazing what Fuji's own processing extracts from this sensor. I reckon the new sensor wiring is responsible for a really low read noise output (hence the ISOless capabilities and unmatched DR).
My research also shows that pretty much all RAW converters that claim GFX support need recalibration when it comes to extended ISO numbers (Lightroom is beta, so I'm not holding this against Adobe, but Silkypix and Iridient already claim official support).
My findings also show that RAWs can seemingly handle a healthy deal of overexposure compared to the in-camera JPEG, which means that you can extract even more DR if you manage to meet the sensor's actual saturation limit, which may well be 1 full stop above the JPEG highlight warning threshold in the camera's histogram and "blinkies" display. Of course, further tests are needed, but that's one reason why I'm flying to Malta Tuesday night.
I am writing up a small article in German (Fuji X Secrets) and in English (X-Pert Corner) that should help illustrate my findings.
