I have tried my 60Hz 1440p monitor overclocked to 80Hz and my FPS V-sync games capped at 80FPS. There is little noticeable difference between that and 60Hz at 60FPS. I returned it to 60Hz because overclocking a monitor above its spec is not good for it long term. I'm sure you know that any FPS above your monitor Hz is wasted energy on the GPU(s).
Regarding 120/144Hz on a 1440p G-sync monitor, you won't get even close to 120FPS with a single 980. I have SLI 970 (each card overclocked to within 95% of 980 performance) and in games like Witcher 3 with ultra settings and 4xAA, don't even crack 80FPS average, and sometimes dipping down below 60FPS in minimum frames. You will need SLI 980Ti's to get 120-140FPS at 1440p with all the eye candy cranked up. Otherwise, you are wasting your money on a 120/144Hz monitor.
Addendum: you will definitely notice a difference in the "fluidity" of gameplay at 60Hz/FPS compared to 120Hz/FPS. I've played around with a 144Hz 1080p G-sync monitor (at 120Hz and 120FPS) and it's indescribably more fluid. Moving the mouse is like moving it through water....butter smooth and no stuttering/jerkiness. Move your mouse around and keep your eye on the cursor...see that micro stuttering? It's gone at 120Hz. You'd just have to experience it.