Efficiency can be a few different things, though. The actual antenna structures and reaction centers are reasonably efficient, at least over certain wavelength

Efficiency can be a few different things, though. The actual antenna structures and reaction centers are reasonably efficient, at least over certain wavelengths. Even through electron transport and the whole photosystem, things still aren't too bad. But overall efficiency tanks because of Rubisco. Even with C4 and CA metabolism, oxygenation is a major limitation.