I was looking for data about halogen reflectors I just bought. They are 50W, 230V.
On the package I read 320 lumen and that looked ridiculous to me, then I found this document by GE: Halogen Lamps Spectrum Catalog.
It states on page 100 that MR16 Start (normal reflectors, 12V) at 50W emit about 680 lumen, while MR16 GU10 (mains voltage) emit at 50W about 340 lumen.
Seriously? 7 lumen per watt?
Now I wonder why this big difference in the emitted light. Maybe because mains are sinusoidal and the lamp absorbs power even when the voltage is low, but maybe the temperature of the filament gets too low to emit properly. Still, this explanation seems odd to me, because thermal inertia should be enough to smooth the \$P=I^2\$ enough.