I once installed a F.S.com Canon S screen in my D800. The calibration was off, so it would have needed-and hopefully been cured by- the supplied polyester shims. I elected not to keep it installed, due to my not wanting to play the Shell Game Of Focusing Errors (especially with plastic shims,) plus the fact that I had visibly scratched it during installation. I never got as far as checking if it made a difference with exposure or AF.
I can tell you that that screen was not visibly course grained in the manner of, say, a F3 or F2/F screen.
If FS.com offered a B screen from the FM-3a I'd try the swap again, but I dislike the split image/microprism gimmick in the center of the K screen.