Dpreview added the GH5 II to studio test scene:
At base ISO there’s no appreciable difference between the GH5 II and the original version. There’s a slight improvement in apparent sharpness, but this is likely to be due to the use of a more expensive lens on the newer model. As with the original GH5 there are hints of moiré, which makes sense, given the absence of an anti-aliasing low-pass filter.
At very high ISOs there appears to be a slight ISO advantage to the newer camera, but it’s not sufficient to make a visible difference at lower ISOs. The JPEG engine appears to be tweaked a little to hold onto a little more detail in low light, though.
There’s not enough of a difference in color response for us to conclude whether this has been adjusted, though there’s a hint of the more muted caucasian skin tones shown by the GH5S. Overall, then, not much difference you’re likely to notice in day-to-day shooting.
Delving a little further in dynamic range, in our exposure latitude test, the deep shadows of the GH5 II’s files look a touch noisier than those of the existing camera, which contradicts the numerical assessments made by DxOMark and PhotonstoPhotos. This could be the result of mis-calibration of the black point in Adobe Camera Raw, which doesn’t expect you to use the extreme deep shadows of images (ie the last few stops of dynamic range), but we’re not seeing the improvements reported when we look at the images.