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Deathadder performs best @ 1800 dpi. 2.5 m/s (450 DPI) – 4.5 m/s (1800 DPI) which you will hardly ever reach using normal movements.
6th notch = NOT ok, because TF2 is a non direct input game. A non DirectInput game is directly affected by the windows desktop and the mouse DPI. This means that if you run a screen resolution smaller than the DPI of the mouse, you will get results of FALSE negative acceleration. This happens because of the way the windows mouse input works, when you hit the boundary of the screen (which you will very fast using 1800 dpi on a 1024 or even small resolution) the mouse will reset to the centre of the windows desktop, which gives a false result of negative acceleration.

Solution for tf2 lower your windows sensitivity.
A nice graph of how bad the deathadder performs @ 450/900 dpi
http://www.esreality.com/?a=longpost&id=1300293&page=3