My experience is that the only non-linear drop in power comes when switching from x-sync to HSS. Typically 1 stop less ‘power’. So changing from x-sync at 1/200th, to HSS at 1/250th results in the appearance of a 1-1/3 stop drop in flash power as the light switches from a single, short burst, to a continuous pulse.
Beyond that, HSS is much like a continuous light source (best to think of it at 1/1 power for this example, but any power below or above x-sync works the same). As the shutter speed increases, the HHS power decreases by an equivalent amount. One stop increase in shutter speed equates to a one stop decrease in the HSS flash power. Where I’ve seen confusion is in the additional loss of ambient. And, that RGB values do not respond linearly to light values.
I tested all this a few years ago by shooting a grey card and reading the RGB values and comparing below x-sync with HSS power and well as continuous vs, flash balance.
Again, it’s been a while, but as I remember it, if you remove the effects of a continuous (ambient) light source, the HSS power is a linear drop. So, I too am confused by the choice of ISO 50. But, I wonder, am I missing something?