Abelcine have put up a quick test of the FS100, testing sensitivity and dynamic range. Despite having the same sensor as the F3, it falls short of that benchmark by a fair margin. Cost reductions in signal processing would have to be the reason here.
The FS100 rates 500 ISO and 10 stops, and the F3 800 ISO and 12 stops, both in REC709 modes. The F3 in S-Log will be 1600 ISO and @ 13-14 stops.
Used carefully though with good lenses, I think the FS100 might make a decent b-camera for the F3. Resolution and colour matching should be closer.