SLI/Crossfire is <mod edit> really, you will be forever slave of whims of developers and from what I see, SLI is not their priority lately.
I used to do Crossfire (and SLI really same story) and many of the games that release and you want to play now have very shoddy CX scaling or none at all, until some patch or driver down the road after you are long done with the game itself.
Of course when it works it's great, but very unreliable really.
That said - 1080Ti is barely above 60FPS in quite a few titles in 4K, so probably one year down the road it won't be enough, but really usually such things boil down to one of the two - either you turn down some <mod edit> filtering or some nonsense feature down a notch (o no, you won't see nose hair moving in the nostril of some dude ingame) or you will suffer the terrible fate of having 50 FPS instead of 60 - the terror.
IMO, it is a brilliant 1440p card for years to come and I think 1440p is also the best choice for gaming for next 2-3 years really, until we have a comfortable range of hardware that actually supports 4k 60FPS with extra breathing room and not just barely to meh.