I used the Fuji X70 and iPhone 6. I'm pondering whether or not I'll get more lenses for the X-T1 or just go back to using Nikon. The nice thing about the X70 rig was the light weight.
I've never been to these shows, but I can imagine the rush in Japanese shows like these, where this kind of compact APSC camera is pretty handy, when it comes to getting the job done. The color rendering is quite good for such a small camera. I see you have the D750, great camera, loads of features, but you might be able get the job done in places like this with just a D5500.
Disclosure- I shoot Canon.
The main thing is low-light ability, or a good flash, or both, and the D750 is best at that, followed by Fuji. Some of the rooms are very poorly lit, as the venue is mainly for wedding parties. What I wish I had was a mirrorless D750, so I took a chance on the X-T1.