I really don't understand why it doesn't use DirectInput (or even better, raw input ala QL/CPM), and use a real in-game sensitivity like Quake/Quake3, etc. can. You can notice the effects on your own PC by putting the slider to 6/11, drawing some curved shapes in pbrush, then changing it to 11/11 and comparing the two. e.g.
TLDR: Don't change in-game sensitivity (unless Blizzard fix it) or your windows sensitivity. Buy a mouse with higher DPI or use a lower resolution.