In the Windows menu, turn off "enhance pointer precision".
Also, disabling mouse acceleration in game is only half the story, you also need to disable it on your system or else it will be present in every game you play. Download the MarkC Windows 8 accel fix (click me)
See if your mouse movement is any different after this.
EDIT: I'd also like to point out that the last Roccat mouse I tried (Kova+) had a glitchy sensor. It's entirely possible that if neither of the things I mentioned above work, it could be your issue.
Also, disabling mouse acceleration in game is only half the story, you also need to disable it on your system or else it will be present in every game you play. Download the MarkC Windows 8 accel fix (click me)
See if your mouse movement is any different after this.
EDIT: I'd also like to point out that the last Roccat mouse I tried (Kova+) had a glitchy sensor. It's entirely possible that if neither of the things I mentioned above work, it could be your issue.