Yup, tried Win 8.1 last night, but without success. I cant get the same speed with 1x 290x and 4x 270x.
I'm back to Win 7 now with 4 cards 1x 290X + 3x 270X).
Dunno why i can't manage to get the same hashrates under Win 8.1.
I tried the same settings as under Win 7, but the latest WHQL and BETA drivers kept crashing...