"Hypothetical: So if a monochromatic LED could convert 50% of its electrical energy to yellow light, it would outperform a monochromatic LED that could only convert 25% of its electrical energy to deep red light. Is this (more or less) correct?"

Only if the plant is capable of using the Emerson effect to downshift that yellow light to another energetic wavelength it can use. Very few plants have this capability, most of them that do have it are aquatic plants. Cannabis is not one of them, it is like most terrestrial plants in that it uses the Emerson effect for downshifting blue into other wavelengths they require since upconversion from red requires an active power input and frequency modulation - something plants just can't do (at least on this planet.)