Adding a little bit more detail to the input lag / mouse lag issue:
Currently DX11 and DX9 handle the mouse differently, and that's why the workaround for turning off V-sync doesn't work. In DX9, the mouse is independent of the framerate if you're in Windowed Fullscreen. So in DX9, your cursor moves the same way it does outside of the game. The game can be choking hard and taking 2 seconds to render a frame and yet I can still smoothly move the mouse.
Regular fullscreen has the mouse slow down with the framerate in both DX9 and DX11. Also in DX11 the mouse slows down with the framerate even in Windowed Fullscreen regardless of the status of V-Sync. This is a problem because it makes aiming at menu items a little difficult and keeping track of your cursor even more difficult. I've been missing ground targeted skills A LOT in DX11.
If you look at other games like The Witcher 3, there is an option called Hardware Cursor that fixes this.
I'm sure this will get fixed.