"BUT you get the job done (photosynthesis) with much less dissipation by running them at high efficiency, so there will be a lot less heat in the system."
Only if the plant can use the light before it dissipates to heat. I've not done tests and I don't grow or smoke pot but I enjoy scientific discussion, and the cheap lights at lower watts spread over the canopy is more efficient than one expensive light in the center pulling the same watts. Is the expensive light so much more efficient that it compensates for this? I doubt it, but only rests on a 4x8 table would show... And I think the cheap lights will have a bigger chance of winning as the watts per sq ft goes up.
Even if they are cooking the diodes and being less electrically efficient they may be more photosynthetically efficient when you factor in the distribution, actual PAR, and the plants efficacy in using that PAR. You really have to look at initial investment costs too since as you said, technology advances rapidly and they need to produce more profit before they ultimately become obsolete.
I'm not saying that the cheap lights truly are better, I'm just saying there are better ways to evaluate.
I still don't believe in the soil/organics either. I don't believe in flushing and I don't believe in lollipoping, I only believe in scientific research done impartially and I'd like to see a double blind study on the taste.