43 rumors posted an article with the exact crop factors calculated for the GH5S's sensor. From what I understand, if you want to use a speedbooster with the sigma zoom lenses you'll be out of luck. I hope I'm mistaken though since I primary use my sigma 18-35mm.
https://www.43rumors.com/exact-crop-factors-caluclated-new-gh5s-multiaspect-sensor/