One thing you have t be a little careful of with the X100 is that in bright street shooting conditions it doesn't take long to run out of shutter speed, particularly if you are shooting wide open. Sure the shutter dial says up to 1/4000 sec, but this is only the case at small apertures. At aperture settings that I usually use most lenses - ie max aperture through to f8, the maximum shutter speed setting is actually only 1/1000 of a second, and in the day it doesn't take long to hit this. I expect that this is why Fuji decided to include a ND filter setting.
Which leads me to another point - is the ND filter just another arrangement to lower ISO? I wonder if, when the sensor is set at ISO 1,600 the sensor performs as if it was at ISO 400. A test will have to be done.