One thing that's becoming apparent and it's rather annoying is that it looks it has significant shutter shock which I didnt see mentioned I any reviews. Initially I thought it's just the higher resolution body syndrome that came with the D800 but my Fuji has a higher pixel density (and the D90 as a matter of fact).
I learned that the 1/FL is decent but I get very inconsistent results with the D750: it works for the VR (but nothing less) but not for the primes where I need 1/2FL to get consistent sharpness. On the X-T1 it works much better.
And I feel the mirror slap much more than other cameras. I guess if I downsize to an X-T1 size the difference will be smaller but overall the high ISO advantage is less than full stop.
It sounds like you're handholding the camera, from your 1/FL point. If that's the case, I don't see how you can discriminate between shutter shock and mirror slap. To see the effect of shutter shock without mirror slap, you have to use MUP mode. If you're hand-holding, mirror slap is almost always the long pole in the tent.
The 1/FL rules was never good for critical work, IMHO. Ansel Adams, in
The Camera (from the second series), on page 116, talks about handholding a normal lens camera -- he doesn't specify which one -- and says that he needs 1/250 second for critical sharpness. On a 35mm camera, that's 1/(5*FL). I've done testing to find out how well I can handhold various cameras and lenses, and I advise you to do the same. Take a newspaper and tape it to your garage door. Back off 20 feet or so. Take pictures at various shutter speeds; lots of pictures per shutter speed, so you'll get an idea of the statistics.. Take a picture with the camera on a tripod in MUP mode for a comparison. See how fast the shutter speed has to be to match the tripod pictures. I bet you'll be surprised.
Here are some test results that show that 1/500 is pretty good with a 58mm lens on a D800E, and 1/250 isn't quite as good..
http://blog.kasson.com/?p=5188Jim