I'll have to think about the math. If filaments were made of the
same wire, they would have to be four times longer in Europe for
the same wattage, but then they would run much cooler. I guess
European filaments are longer and thinner. How does that require a
less energy-efficient design?

My understanding is that standard incandescent lightbulbs are most efficient
at around 100V, so US bulbs should work better than European ones. There's
basically a most efficient thickness of the filament. A 230V filament is too
thin.

Christian.