Again I haven’t done a complete study, but raising them side by side in the same brooder it’s easy to say that there’s a better conversion. Really what you’re getting is a quicker turnover and a heavier breast. If the Brahma chick eats a teaspoon of food, the Cornish will eat a cup.
I'd love to see your numbers if you ever do a comparison. I know cornishx are bred for feed conversion and they are exponentially more efficient in converting feed to meat than standard dual purpose birds. I believe the short growth period is sort of a by-product of good feed conversion.