Is the Fury X running into a memory wall at 4K?
Considering R9 390X shows no advantage over 3.5-4GB cards, I am not sure why you'd say that.
Looking at this game's textures (i.e., average) and the minimum frame rates of GTX780/780Ti SLI setups with 3GB of VRAM, even 3GB isn't a bottleneck at 4K.
The graphics are
very unimpressive. Compared to The Witcher 3 or Rise of the Tomb Raider, this looks like a last generation game, not a 2015 game.
This game screams console port/outdated game engine.
Even on a Core i3-2100, CPU load barely gets to 50%.
Then again, this game wasn't meant to push the PC at all and cards like GTX960/680/7970 can get to nearly 60 fps at 1080P.
Surprisingly at 1600P, cards like 780Ti/290X/980 are hovering near the 30 fps min mark. I am personally not impressed with that given the graphics I am seeing.