If we see that the various photoreceptors are at work creating the Mcree curve it would not be difficult to explain the shape which is a mix of symmetry and asymmetry. The 580-680 range of the Mcree curve isn't exactly smooth, but the relative deviation is minor, even for the 660-680 range though it does poke out a little bit. Beyond that it falls back in line. There's also an odd linear region between 475 and 525.
If we superimpose a random plant photoreceptor response graph onto the Mcree curve we get some interesting results. I'm not suggesting either of the two data sets are perfect for a particular plant, but to some degree they should be close right?
This might suggest that all other things being equal it would be useful to target a handful of wavelengths. The individual receptor response curves explain the Emerson effect. Since LED contains almost no 400nm light there's especially good reason to add light in the 660-690 region, 675 specifically so chlorophyll A is active. There are also 4 phosphor curves in the pic, 4000K 80CRI, 4000K 70CRI, 3000K 70CRI and 2700K 80CRI. The 4K SPDs ave been compressed so the 400-500nm band is in the 10-12% range. The other two samples were compressed at the same ratio for comparison but might benefit from some additional blue.
For a simple 2 part solution (cob+660) I would probably go with 5000K 70CRI or 6500K 80CRI. Those SPDs would have been more difficult to carve out of their respective graphs so I haven't added them. I think 4000K 70CRI would work well enough using 660nm to lower the K to 3000. If we use 660nm light to lower the 3000K sample to 2700K that would provide about 5% blue, which is similar to traditional HPS spectrums.
Another approach would be to target specific colors and improve on red/blue, which might prove to be more efficacious if done in the correct ratios. The target wavelengths, going by the chart below would be 400 (or 675), 440, 575 and 625. I would hesitate to guess on the desired ratios. I'm also not clear on why chlorophyll A is showing a stronger 400nm response as an individual photoreceptor when it seems to create more photosynthesis in the Mcree curve at 675, or whether 400nm light would invoke the Emerson effect the same as 675nm light does. Any ideas or consideration on the subject?
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