HiTech Legion tried the NVIDIA GeForce GTX 1080 vs Titan X vs R9 Fury vs GTX 980 Ti vs GTX 980 vs R9 390X A quote from the article:

The NVIDIA GeForce GTX 1080 not only outperforms the GTX 980 which it will be replacing, its performance eclipses that of the Titan X which until now was the performance king. When I ran my benchmarks I was actually shaking my head in disbelief and re-ran the suite for a fourth and fifth time just to make sure the performance increases were correct and I was running everything at the same settings. So how could I not do a vs video? I haven't seen and don't ever remember seeing an increase in performance in one generation.

 NVIDIA GeForce GTX 1080 vs Titan X vs R9 Fury vs GTX 980 Ti vs GTX 980 vs R9 390X @ HiTech Legion