I still have to test the corner and border performance of the Pentax 645 FA 150-300mm ƒ5.6 IF ED zoom. I sometimes see a sort of double jitter sharpness (double contoured details) at the borders. But since I have not taken any image that are "flat", such as landscape images where the whole frame is supposed to be sharp at infinity - which would then reveal if there are any issues at the borders and corners.

Thick Sensor stack usually interferes with optical performance

Remember; Fujifilm GFX cameras have excessive thick sensor stack [glass layers]; in total 3.26 mm excluding air in between the three layers. This can distort vintage glass, in ways that it shows especially in the borders and even more so in the corners, with all kinds of possible effects; smear, jitter-like sharpnes, stronger chromatic aberrations, blurriness, etc

So - that, i still have to investigate.

Impressions

From what I can tell is, that the 150mm focal length shows better optical performance with the FA 150-300/5.6 IF ED zoom compared to the FA 80-160mm/4.5 at 160mm ƒ4.5 - which isn't fully as sharp and has more visible chromatic aberration (less in the 150-300mm lens at wide open aperture). It is likely that the ED elements in the longer zoom resulting into better optical performance. I don't think the 80-160mm zoom has any ED glass. It would otherwise noted in the lens name.