It will be unlikely for NVIDIA to release any more high-end cards this year, they simply have no need to do as they still cannot meet demand on 970 and 980. Once they do meet demand all they would need to do is release 8GB version to start the massive demand all over again.
8GB on a 256bit bus is going to cause quite a bottleneck especially at 4k Resolutions. I would imagine anything with 8GB having a 512bit bus tbh but it's Nvidia so who knows......