Newsshooter had the opportunity to test the Metabones Speedbooster XL on a pre-production GH5 and while the video results look good there are some problems. At this time the electronic connection between lens and camera isn’t working correctly and if you use IBIS there can be issues, but other than that the results look very good. Check out the full article at Newsshooter for more details.
Panasonic GH5 B&H photo/Amazon/Adorama
Metabones Speed Booster XL 0.64x B&H photo/Amazon/Adorama