I think so far the experiments are shaping up to confirm the original hypothesis, that the actual size of the root system, doesn't have anything to do, independently, with the size or healthiness of a plant. As long as the plants are getting the nutrients that they need to maximize the amount of light they are getting too photosynthesize. It would make sense that the plants in nature have large root systems, seeing as they have pretty much all the light they can handle, and that in general each cubic foot of soil has a set level of nutrients in it (varying by region of course), so in order to find more nutrients to maximize the sunlight, it has to have as much root surface area as possible. I think FDD's outdoor plants are also a great example of this, his plants are massive compared to his pot size, because he feeds them enough nutrients. I have also seen plants (my own) which did not have enough light or nutrients, but were in very big pots, and the root system was larger than the upward growth.
I would be interested in a side by side comparison of 2 plants grown outdoors in the soil, one which just gets rainwater and watever nutrients are found in the soil, and another which gets a good feeding of nutes right around the base of the plants. Then when the grow is over pull out the root system to see what the size difference is (if any).
p.s. on a side note about the B-vitamins, Ive been looking at experimenting with my GF's pre-natal vitamins, which have 4 or 5 B-vitamins in them. I don't know if these are the right kind, but if so, they could be a cheap and easily available additive.