Check out this test made by Robin Wong ;)
Dcomark tested the GH5II sensor performance and concluded:
The original GH5 had been showing its age lately, and while the launch of the GH5 II was overshadowed somewhat by the announcement of the GH6, the GH5 II as an interim model sees some welcome updates — and not just to its video specs. Although the sensor appears to be similar to the original, the addition of an AR coating has resulted in enhanced transmission by reducing reflection. This in turn appears to have improved the overall sensor SNR, leading to welcome gains in color sensitivity and dynamic range, notably at high ISO settings. Couple that with a lower list price at the time of announcement compared to the original model and the GH5 II seems like a highly compelling update, and that’s regardless of the incoming, more pro-oriented GH6, which is sure to be much more expensive.
CameraLabs reviewed the new lens and concluded:
The Olympus 8-25mm f4 Pro is a fresh and very welcome addition to the Micro Four Thirds system. There’s lots of ultra-wide or standard zooms in the catalogue, but few that effectively cover both ranges. The earlier Leica DG 8-18mm started very wide but often left you wanting to zoom a little longer. Then the Leica 10-25 sacrificed a little at the wide end to extend further while delighting with its bright and constant f1.7 aperture, but this made it a heavy and expensive option for most of us. With the 8-25mm f4 Pro, Olympus has successfully filled the gap between the two Leicas, starting as wide as one and ending as long as the other, but opting for a more modest f4 aperture to keep the size, weight and cost down. The retracting mechanism also allows the lens to shrink to a compact size without getting in the way of operation. The result is a lens that delivers a very flexible range from ultra-wide to standard coverage with useful close-up focusing and a compact size. Sure the f4 focal ratio on Micro Four Thirds means you won’t be enjoying shallow depth-of-field effects unless your subject is very close, but that’s about the only sacrifice to achieve what’s arguably one of the most compelling walkaround lenses in the system. If I had to choose just one lens for general use on a Micro Four Thirds body, the Olympus 8-25mm would be it, and it comes highly recommended.