The GTX 1080 benchmarks have been leaked and show some nice performance improvements over the current generation for cards from Nvidia and AMD.
Show below are performance numbers from 3DMARK 11's Extreme and Performance modes and 3DMARK's Fire Strike modes for Nvidia's GTX 1080. The GTX 1080 is compared to current high-end cards from AMD and Nvidia from the GTX 970/R9 390 and up to the Titan X.
On average the GTX 1080 is around 2x as powerful as a GTX 970 or an R9 390. The current favorite the GTX 980Ti is around 25% slower than the GTX 1080.
Of course, these results are leaked and not yet confirmed as genuine.