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

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 AppleScript code 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"


EDIT: 
    defaults find mouse
will search your default domains, keys, and values for the word mouse, without the need to pipe to grep.