Scaling works for all linked layers. You would think scaling up all layers would increase all line widths 
After scaling larger you can use the crop tool to reduce the size of the layer group back to it's original size.
Another method. If you have G'MIC-Qt installed so it is used by Gimp you can use Artistic > Graphic boost to increase line widths and then 'apply to all layers'. This means you don't need to pack layers.
After scaling larger you can use the crop tool to reduce the size of the layer group back to it's original size.Another method. If you have G'MIC-Qt installed so it is used by Gimp you can use Artistic > Graphic boost to increase line widths and then 'apply to all layers'. This means you don't need to pack layers.