Ok received the Zhongyi lens turbo yesterday along with a normal nikon to m43 dumb adapter to use with the nikon sigma 18-35 with the GH5.
First impressions - No issues I can see at all with Vignetting and if it is there it is not noticeable, at the wide end with the Zhongyi which is around 13mm after conversion and it seems fine.
In terms of image quality I am finding it hard to see any noticeable difference between the image out of the Sigma through the dumb adapter or the Zhongyi which is great news to me. I shot some stuff in doors and saw comparable quality in terms of sharpness and resolution throughout. The zhongyi might be more prone to flaring when pointed at a light source but that is avoidable..
In terms of the IBIS and focal settings you have to input every time you turn the camera on. I think you pretty much have to stick to the focal length setting that relates to the focal length that you are going to use on the lens. So if you set it at 18mm, shoot at 18mm and you will get good stabilisation. if you set it at 18mm and then zoom in at 35 you get strange warping effects and issues. there is a delay from when panning to when the sensor \stabilisation recognises the pan and then catches up. if i set the focal length in cam to 35 and shoot at 35 again this looks great and stable, but if i then choose to go to 18mm i get the same issue, strange warping effects and delayed panning.
The issue is the focal length setting in camera can only be changed when you are not recording. this means you cannot while filming both change your lens focal length and the in camera focal length setting. You basically have to choose what focal length you will be shooting at before you start recording and stick to it unless you want some adverse effects....
If you had to stick with one setting I would say you get better results going with the widest setting, so for the sigma you can set it to 18 and get great results at 18 and then the performance gets worse the more you zoom in. I think this option is better than setting the focal length to 35 which then only gives you great results at the far end, and then degradation the more and more you zoom out.
all in all in terms of the Zhongyi...I would not hesitate to recommend it as a very cheap alternative to the speed booster.