I recently participated in a workshop with a very knowledgeable professional. He has switched to Fujifilm cameras after having been a long time Nikon user. I am also a long time Nikon user who is now moving over to the Fuji X-T1 and enjoying the transition. My main motivation is to reduce the weight and bulk of my kit without giving up on quality.
This professional advocated that ISO stops of 160, 320, 640, 1280, 2560, 5120, etc will give reduced noise in the dark areas. Since the X-T1 is native at 200, he advocates setting the iso to a base of 320. He also said that this applies to Fujifilm, Nikon, Sony and Canon. He said it did not apply to Olympus (I don't know why). He played a test video showing the results. Since that workshop, I have been trying to get additional information and, I have to say, that it is confusing. I was hoping that a Fujifilm expert could help in this matter.
Please Google "iso 160, 320, 640, 1280" and you will see numerous articles discussing this subject. I have a feeling it is beyond my pay grade but perhaps someone in the forum can help with the explanation and recommendation.
Also, I would think that someone at Fujifilm should be able to verify or debunk this. Just getting to that person is very difficult. I look forward to the input of this forum.