Where does OS X store mouse preferences?
The tracking speed is stored in ~/Library/Preferences/.GlobalPreferences.plist.

$ defaults read -g | grep mouse
    "com.apple.mouse.doubleClickThreshold" = "1.1";
    "com.apple.mouse.scaling" = "2.5";


The slowest speed shown in System Preferences is 0.0 and the highest is 3.0

You can change the speed by editing the plist directly or with defaults, but you have to log out and back in.

defaults write -g com.apple.mouse.scaling -float 2.1


This would change the speed immediately:

tell application "System Preferences"
    reveal pane "com.apple.preference.mouse"
end tell
tell application "System Events" to tell process "System Preferences"
    set value of slider 3 of window 1 to 9.0 -- maximum, minimum is 0.0
end tell
quit application "System Preferences"