Since the Ag departments at universities are generally the ones who do controlled studies on feed conversion, and dual purpose aren't raised for meat commercially, you are probably going to have to get that information from individuals.
You won't get a true feed conversion rate, just what one person got with their birds under their growing conditions.
Your very best option is to do a controlled study for yourself with the breeds that interest you and under the conditions that you use to raise your own meat birds. Carefully weigh each serving of feed and keep track of which bird is eating it. Weigh the bird often.