I have a 4k monitor with two GTX 1080 in SLI and I usually don't get 60fps if I play on max settings. Annoyed by this I got another monitor at 2560x1440 @ 144hz and I can't max that out either. The games were stuff like rise of tomb raider or witcher 3.
I have a gsync 1440p 144hz, running on a 980ti. IMHO, if you can get at least 90 FPS it's a great experience. I don't really see the need for faster frames.
Granted, I believe gsync does a lot to make it appear smoother so if you don't have that it may be more important to max the frame rate. I do see it though when I drop to 60 fps, it feels choppy.
It's still a silly situation to be in, you spend all that money on a fancy monitor and GPU, yet you can't even max out visual fidelity if you want to make full use of them.
About GSYNC: Afaik isn't its purpose to eliminate screen tearing without the use of vsync, thus not getting stuck with awkward halfing of fps if the GPU can't keep up?
I think 1080ti situation is similar.