The reason for the increased VRAM has nothing to do with needing a larger frame buffer. For some time now, games have been using VRAM more than just a frame buffer. You can see this in games like watchdogs. Even at 4k, you would not need 8Gb of VRAM. It has to do with the fact that VRAM is now being used as storage instead of just RAM. This trend is going to increase, as the consoles used a unified system memory structure and ported games will want to use VRAM.A very sneaky move by AMD. Adding more VRAM to add more value in order to buy some more time to sell more cards and fight the Maxwell enemy. I'm not sure if Hawaii itself can even utilize an 8GB frame buffer but this surely does add a lot more value to AMD's side again. This especially being the case with game developers putting the smack down on VRAM and gamers wanting to move onto 4K which will result in the obvious need for more.
The 980 isn't miles ahead of the 290X in performance so this is an excellent comeback until the 300 series is completed.