I put Kyronaut under the lid of my CPU and between the heatspreader and CPU cooler. Seems to do the job. Their claims of longevity will be tested there.
For the money you can get a lot more Arctic MX-4, and only sacrifice a little bit of performance on CPUs. Works well on filling in large gaps on direct contact heatpipes. But if you look at the GPU numbers Arctic MX-4 doesn't do so well, but Kyronaut keeps it lead, though I have still used on GPUs.
I actually don't recall what I used on my GTX1080, and it didn't make it into the pictures. Might have been whatever XSPC supplied with the water block.