No...I think you missed the point. If the object shows a width of 400 in the GUI it will trace 404 pixels. Here is what is happening:
For objects that have a border, the object's actionscript width (what the object.width value with trace) is equal to the object's visualWidth (what is set in the GUI) + the object scaleX. It's a bug!
Look at the attached FLA and notice the "track_mc" with set in the GUI and then notice the width that it traces. They are inconsistent and wrong. The trace value
track_mc.width = trackGUIwidth + track_mc.scaleX;
It is consistent among all objects with a border.
What I need is to be able to set the visualWidth with actionscript. Setting track to 300 pixels with actionscript will actually make it display at ~297 pixels.