I found what I wanted in another thread. Someone posted this from a Citizen pdf. As you can see, the 5000k has a slight advantage over the warmer whites in PAR per watt, though actually it's only about 5% at most. Still, since it is the most efficient at delivering PAR and is also closest to the CCT of sunlight at noon, might as well use that one. The sun is about 5500k from what I've read. The higher CCTs are said to give frostier buds anyway and shorter plants. Doesn't seem to be a downside really.
I don't think it actually matters much what the exact spectrum is, just the total PAR. For instance, I've read that plant growth increases with more blue up to 50%, and none of the white COBs have over 50% blue. I think you only need enough red to activate the phytochrome based flowering system. In fact, I've seen experiments where you can use pure blue as long as you also run some pure red for a few minutes after the blue is turned off. With pure blue the flowering is prevented, at least in the plants they were using, presumably it's similar with weed. I've even seen experiments where they grew vegetative plants with pure red, blue and green light and blue always comes out ahead in dry weight. Surprisingly, with pure green light they only grew a little less than blue or red, but it was always the least of the three.