I wondered about them too, For me I had stuttering that resulted in minus minimums and many on screen textures missing, One run through had the whole floor texture missing and it's not much better in game, Or at least it wan't when I did my playthrough (2nd patch included). Some maps where unplayable even though the fps was good the stutter was constant. With high texture settings it was smooth as ice.
I suspect the problem with the Fiji cards is more to do with the game devs rather than the fact that they have 4gb of memory.
I will find out how my 4gb 980s go on the bench in a minute.