After a little tiff with some angry man over on Reddit, I had to go conduct a few experiments of my own regarding the effectiveness of green light on photosynthesis.
To get to the meat of it all, between ~450 - 1800 umol, green light tends to drive photosynthesis more efficiently than red and blue light, especially in plants with much thicker leaves, like actual trees, succulents, cacti, etc. A tiny bit higher than 1800 umol, and your ROI drops rapidly as you've pretty much hit saturation of the chloroplasts, for all ranges of light. The law of Diminishing Returns comes to bite us in the butt, and no wavelength is ultimately more efficient than any other, and in fact, you begin reaching damaging levels of photon saturation which can burn a plant.
Now, there are multiple food crops that have an optimal light requirement below 500 umol. Multiple types of lettuce grow very well with ~200 umol, for an example. For that purpose, LED lights using red and blue light do well more than simply suffice. For rooting cuttings, red and blue light works very well. Fruiting plants typically require much more light, Jalapenos like it around 1500 umol. I've grown nice-sized jalapenos, bigger than what I'll find in most supermarkets, under a red and blue only panel, but I suspect I might be able to do a bit better with some focused green light. I'll do that when I get one made with equivalent specs.
Of course, getting to that intensity for larger plants will likely require me going solid white with a carefully-tuned color temperature. I have seen units now that on equal power draw with their HID counterparts are beating them by roughly 20%. However, these still require either EXTREMELY LARGE or very exotic cooling systems for anything over 300w and in a reasonably small package, and are not totally feasible for either green house or consumer gardening, this will likely be stuck in the parking light/street light arena for a while.
I have seen the benefits of green light, but there isn't MUCH additional benefit versus splitting up into different LED modules so you can get the light deeper into the canopy with the red/blue units. Perhaps for those doing bonsai, or growing cacti, and possibly underwater corals/plants, there is a good use, I can see it. Well, at least I'm sure of why an HPS works rather well despite having a very 'poor' spectral output. It's more than just the 660-680nm + IR that it dumps in massive quantities, it is also the green light having its own effective role.
Kinda goes totally against the conventional knowledge that red and blue are the most efficient, eh?