Well, I can say that with my experience with a GTX 1080, it only likes to work harder when necessary. I have a 1080 and 7700k and my GPU usage without shaders is always pretty low. As long as the FPS doesn't drop below your monitor's refresh rate, you should be fine. I have a 165Hz monitor and the GTX 1080 never really works harder until it dips below 60 FPS, and I don't know why. I have no idea why your 6700k would be at 40% usage unless you have background processes open. Anyways, as long as you are getting above your monitor's native refresh rate (I assume it is 60Hz), you should be fine.
Also, when there are very many mobs in one place, your FPS can drop substantially. Especially if those mobs have animations (blazes). From what you said, it sounds like there are only a few blazes. Based on that, I assume your GTX 1080 is being a bit lazy (like mine lol).