Atmosphere reduces solar flux by 1/10 or so. That's a greater advantage than the day/night benefit. Can't speak to the ISS panels - are they old tech? Ultralight design limited their efficiency?
The panels must be old tech but that in itself would be a problem unless we see solar panels leveling off in their efficiency in the near future. Whatever you put into orbit is going to become outdated quickly. This is the source for the amount of power generated for the ISS if you are interested.
https://www.edn.com/design/power-management/4427522/Internat...