no magic there ... classic HSS will get you even illumination when you go above X-Sync, but a big loss of power because it is not one impulse, but a lot of small impulses... HS is one impulse but going beyond X-Sync means you have a traveling slit and impulse is not even ("grading") across its duration and for GFX50R that duration (beyond X-Sync no matter what is the slit it will travel across the frame still in X-Sync time... even if the slit is very narrow and you have 1/8000 exposure time that narrow slit will still cover sensor in X-Sync time) is X-Sync time of 1/125 (actually a little faster, but let us stick with nominal numbers). I do not think Quadra HS head can keep a single 100% power impulse very even for 1/125 sec, it might keep it sufficiently even for dSLR/dSLM with 1/200 - 1/250 X-Sync times, but 1/125 shall be a a challenge... did you see a tech spec about t0.1 & t0.5 durations (actual numbers) of 100% power impulse for HS head ? - albeit some say that their strobe models can make it almost not noticeable or correctable in post ... if you can't keep yourself within X-Sync shutter speed (to avoid grading) and kill ambient light outdoors using either aperture and/or ND filter then it is a tough luck then ...
https://www.elinchrom.com/learn/hss-hs.html
