The command console is locked out/removed. Have a look in the GamerProfile.xml in My Documents/My Games/Far Cry 3.
You could try changing Sensitivity="1" to a minus number but I'm sure that would work. Try 0.5 instead or whatever. If that doesn't work change your mouse speed settings in Windows, if that affects the game - not sure if it does. And if that doesn't work buy a new mouse with a DPI switch and set it to the lowest or whatever speed you're comfortable with.