The only game that I play, that is playable at 4K on a 1080Ti is Elite Dangerous.
Battlefield 4 & 1, Destiny 2, The Division, PUBG, COD WWII, and Tomb Raider are not playable at 4K for me.
When I ran 2 GTX980Ti's those games at 4K were just about playable but some of them I still had to run at 1080p or 1440p.
There was some micro stutter that I noticed, and in the likes of The Division SLI made no difference.
If nVidia and game devs put the effort in to get SLI working, then it would be great but to be honest since most card's coming up are going to be capable of at least 4K 60fps with high if not max detail, I don't see the point in it anymore other than for benchmarks.
Not to mention most people still only run 1080p or 1440p monitors anyway, so SLI is not really worth it for that.
Now VR is a whole other ball game, but it's still not something that everyone uses or wants to use, but if they could get it so that I can run Elite maxed at in my Rift and have a solid frame rate in space, stations and on planets then I would be happy and would use it more other than only when exploring.