I'd love to see a study done using a small species of parrot, a medium species of parrot, a large species of parrot, and eclectus parrots (cause they are that special!).
What I'd like to see done is to take pellets from each company, and feed these pellets to a controlled group of birds... At least 3-5 per species per pellet - with each species determined healthy via bloodwork before this study. Each group of birds would only get one pellet to eat, nothing else. I would then like to see the results of bloodwork after 6 months, 1 year, 3 years, 6 years, etc.
So lets say we are testing out Pretty Bird, Kaytee, Harrison's and Zupreem. We are studying the effects of those pellets in budgies... 3 birds per pellet, 4 different pellets, 12 birds. If also studying within other species, that might also include say 12 cockatoos and 12 conures or pionus or pois. Minimum, we'd be looking at 36 birds for this study.
Of course, there are a lot more pellets than those, and it would be best done with 5-6 birds per pellet at least... so hundreds of birds...
If done, this might help weed out the better pellets from the lesser ones and perhaps determine the best pellets (in general) for each species - but not the best pellet for an individual bird.