The higher the refresh rate, the smoother the game (if your GPU can drive it that high).
I too have a 980 Ti with a 144 Hz G-Sync monitor and you can see the definite "smoothness" at higher FPS than lower FPS.
As for stutters, CFS3.exe will stutter (drops to zero and then back to 60 FPS) normally. You really get to see how much, if you use G-sync, and see how many times it "flickers" from when it drops to zero and back up in the background. Using a normal monitor, you do not really see how many times it does this, but the way G-sync algorisms are written, it become very obvious. But the higher the refresh rate, the less noticeable the "stutters" are to the eye.
Having your monitor at 144 Hz will not help during time compression, that is all CPU calculations, not GPU, since "what you see, is what you see" as far as the GPU is concerned when you are flying. Time compression on the other hand, means that the CPU has to calculate everything going on in the background at an increased speed (2x, 4x, 8x, or 16x) which can really drag the FPS down.
