mcCursor._x = (speed * _xmouse) - (Stage.width * speed / 2) + (Stage.width / 2);
mcCursor._y = (speed * _ymouse) - (Stage.height * speed / 2) + (Stage.height / 2);
Previous code was wrong.
Also as you pointed out, the minimal speed ratio has to be 1. Anything lower than 1 will make your cursor run out of the stage before the movieclip reaches the edge. The thing with first person shooters is that the cursor is always forced to the center, eliminating the problem. Flash cannot do that.
One work around is to enable moving the stage with the arrow keys.