http://www.hardocp.com/article/2015/08/25/witcher_3_wild_hunt_gameplay_performance_review/8#.V1xyp4-cH8w
Hairworks Low for the GTX980Ti was 13% drop, and High was 21% drop.
Your game will run about 27% faster at 1920x1080 than the 2560x1440 resolution.
If you look at the benchmarks you'll see that it's not above 60FPS, so if you'd like to use VSYNC (again, maybe Adaptive VSYNC) you'll need to adjust several of the settings such as AF (use 4x or 8x instead) etc.
Frankly, it may be too demanding depending on whether you use VSYNC ON or OFF (or had a GSYNC monitor) so it may not be worth using. Again, I don't own the game and there may be other settings you can drop that don't add a lot visually to get to whatever your FPS goal is.
Other:
I know MONSTERS have Hairworks as well, so I'm a little confused by this (Is Geralt's hair separate?)->
"The subjective part also comes into play though. At this lowest mode of HairWorks we really did not experience much of a noticeable improvement in Geralt's hair in this game. Visually the performance drop for enabling HairWorks doesn't balance out. You might as well just disable HairWorks and run the game without it, you really wouldn't notice a difference visually. The hair still bounces with HairWorks off. "
Is LOW for Geralt only, and HIGH for him and the creatures?