I can't disagree with any of your observations Steve, but I've not fed the sort of group sizes that you have. The logic you've applied to the situation seems sound.
The group of 2 checkereds and 2 blacknecks I fed together had resuvthat sort of support what you observed. The female blackneck started eating better after Binky and Eskerina started becoming more "enthusiastic" and had a couple of fights. It's a single occurrence, so not significant as I can't rule out other factors or a simple realisation in Sybil that she needs to eat quicker.