I'm not surprised to read you're finding it a bit choppy at 4K res - I think a single 1080Ti isnt as hot as it was made out to be! I wouldnt want to run 4K on a single Ti, its only just about where I want it to be for 1440p.
Also i think your rig is more than adequate..I wouldnt be upgrading it to a Gen 7 Intel rig myself! I was thinking about upgrading mine but when I looked into it I doubt I'd notice any difference and decided to wait for Coffeelake next year!
I can easily get 111fps on 4k on ultra in PVZ GW2 now with my 1080ti, probably not the most demanding games but with my Titan I was not able to run smoothly at 2560x1600.
I must admit, at first I felt a little let down but now I havbe all the bugs ironed out I am VERY VERY impressed with it.
Now I just have to decide whether or not to upgrade my ram to something a bit quicker. Would it help going from 1600mhz to 2400mhz?