I'm actually someone who has the setup which should create all the difficulties:
2 GTX 970 in SLI, connected to the Philips 40" 4K monitor.
I can report the following frame rates from playing these games:
Far Cry 4 - 4k, everything on ultra, 2x AA: between 30 and 60 FPS. No stuttering (but other artefacts, which I blame on the game but not on the card, as Far Cry is the only game where I see them.
Dying Light - 4k, everything on high, AA off, as AA on produces horrible texture flickering (again, that's the game, not the card). Between 30 and 50 FPS, but some stuttering when you turn around quickly. It's very playable though.
Dragon Age Inquisition - 4k, everything on high, 2x AA, between 30 and 60 FPS. No stuttering, except when you turn around quickly. Very playable.
I was considering getting a second GTX 780 to the one I used to have, then got the two 970s. Only when I had ordered them did I read about the "bug" and got worried. However, these two ASUS cards are silent in desktop mode and use a lot less power. I didn't bother trying everything out with the two GTX 780. I doubt I would have got better results from them.
In case you're wondering: I'm keeping these two cards as I'm very happy with them.