I wonder if the smaller AF area is due to light falloff or lower contrast, as you head away from the center of the lens.Forced to shot with higher ISO is only one of the down sidea of these lenses.
Seams to be limitations when it comes to AF also as there is a reduction in overall focusing area...down to a 60 x 40 grid, which is similar to the autofocus area of the Canon EOS 6D Mark II.
AF speed will probably be fin in day light/good light but how will it perform when the light fall or you are in wood lands?
How will the AF be on the RP and R ?
Otherwise I can't see how the lens would have that kind of limitation. But then I'm not an autofocus engineer either.