Thanks @krucible,
I think you have a point there. While revisiting the topic I linked to above, I found that the horizontal fader has a maximum size. So scaling the GUI up messes up the alignment of the drawn keyboard 
(which scales well) with the length of the fader, which simply stops getting longer at some point.
It’s definitely worthwhile designing the GUI in as close to full screen as the edit mode allows. Not sure yet what happens when you go to live mode on laptop screen resolution after having designed the GUI on a 4K desktop monitor.