This is an intended feature but works too unreliably. - This setting was stored in each face and only editable by re-running the unwrap, this is too hidden (only discovered this by reading code). - This worked with blender internal but not with cycles, such basic options as changing an image shouldn't behave different depending on the render engine selected. I've ifdef'd out the aspect correction for now, it could be added back as a per scene option and be made to run on both cycles or blender internal but for now I prefer to keep this disabled.