I was using Focal 1.7 Pro production release with Windows 8 Pro x64 a couple of days ago. The focus peak feature was producing odd results with my 7D, which I've reported, and a did get a couple of glitches with my 5D3, which just needed me to restart whatever I was doing in order to fix. I don't think that's unique to Win 8 though, so while it may not be tested/certified it does at least run and will produce calibration results which look right to me.