I find the X5 works well up to ISO 800. Here's an example:
This was a very challenging situation because, as you can see, the lighting is very dynamic and the dynamic range of non-RAW D-log leaves a little to be desired. There were no rehearsals. No practice. I got one crack at this. So I was hyperaware of over-exposure. I didn't want to blow out the NM sky, but at the same time I wanted the balloons to be visible even if they weren't running their burners. I obviously didn't want grainy footage, either. So I just set the camera and go. I chose ISO 400 at first but I think I kicked it up to ISO 800 as the sun dropped under the horizon. Shutter was 30 FPS (consistent with the 360° shutter rule....LOL.....). I started with the aperture slightly closed f/4.0 I believe, and then opened it up as things got darker. The reason for this is that you can't "rack" the aperture, so running through aperture presets is much more subtle than jumping from ISO 400 to ISO 800.
Bottom line; the X5 did pretty good. Obviously, this would've been much better shot in RAW. But alas, I do not have an X5R camera.
Nice. You are lucky on the FPS. I recently had a project related to a rodeo that started near sunset. The activity in the arena needs more than 30FPS. I barely made some of it with 800 ISO. I ramped it up to 1600 and that was really on the border. It does help to shoot 4K. It gives some more data to work with. Still tough.