Fujifilm have released a firmware update for the X100S.
The only fix is listed as follows
1.The phenomenon is fixed that the brightness of the OVF bright frame doesn’t change according to a surrounding brightness in case of ISO 1600 and higher.
Sadly it doesn’t include anything about AF being improved at low light that has been reported (by myself alongside others now) in low light.
Get the update here. Installation is easy as always – copy it to an SD card and then press and hold the Back/Menu button whilst turning the camera on and following the instructions.