And to be fair other cameras also suffer shutter shock and its unlikely to happen at this focal length / shutter speeds.
From all the reviews and testing I've read, the phase detect AF on the Oly EM1 works better than the DFD for tracking subjects, certainly on my EM1 the tracking is as good as I've seen with any lens on any mirrorless system (I was tracking stunt horses in an arena and basically every shot was in focus at 9fps). In SAF the focussing is instant and more than quick enough.
The EM5ii might be a different matter as it doesn't have phase detect but a recent review on Steve Huff's website suggests that focus and tracking is fine with that camera too (at some point I'll try this lens on mine...).
I guess it depends on what you're shooting but I think a more sensible comparison here is the EM1 vs. GX8 as they are both the 'top of the range' models with 'best af' designed for shooting moving subjects and are more comparable size and feature wise too.
In the end, I wouldn't let AF nor stabilisation make the decision here as unless your technique at 800mm is amazing that will certainly be the limiting factor here. Choose based on form factor and other features.