Update on this for future reference: using multi-junction solar cells, the theoretical ceiling for efficiency is about 68% [1], and we can probably assume LEDs will get up to about 90% efficiency in producing desired wavelengths. Thus in an ideal future scenario it might be possible to do better with solar collection + LEDs than plants themselves do, but we're pretty far off in actuality.

https://en.wikipedia.org/wiki/Multi-junction_solar_cell