GDDR6, which is said to be 30% faster than the currently popular GDDR5 memory, will now roll out at the end of this year, but it's not clear when we'll be able to find them in our GPUs.
http://www.notebookcheck.net/Micron-is-accelerating-the-release-of-GDDR6-graphics-memory-to-meet-demand.195652.0.html