I have a Fury-X and game at 3440 x 1440 and personally haven't noticed any issue with vram capacity, the card runs out of grunt before 4GB is a problem, atleast in the games I play. Whether that's down to driver witch craft or an inherent advantage of HBM i'm not sure or maybe the games I play just don't need more than 4GB(?)
In some modern games the FuryX has dropped in performance a long way behind the 980ti when at release there was only a small difference.
for example mass Effect:
http://www.pcgameshardware.de/Mass-...55712/Specials/Mass-Effect-Andromeda-1223325/
980ti is 46% faster.