Recently I rented a EF600/4L II and had it optimized. To my surprise it works incredibly well on X-T3 when doing AF-C tracking.
I set X-T3 to AF-C mode, CH (11 fps) and Face/Eye detection on. The focusing object is a person riding a electric motorcycle moving fast towards the camera.
I also tested video mode Face/Eye tracking. Thanks to the great improvements of PDAF performance and coverage of X-Trans IV CMOS, both still and video mode works very well.
Combined with the video clip recorded from the HDMI output while doing continuous shooting testing, I made a demo video.
The support for EF600/4L II would be added into the next version (V2.50) of Fringer EF-FX (Pro) adapters' firmware which is to be released soon.