Not only this, but their performance is going to be entirely dependent on which specific cross you get. A Ross CX is going to be different than an Aviagen one, etc. Most people don't know exactly which ones they have. These birds are the product of a planned cross between several different lines that have been linebred for decades. The cross is set up to produce specific meat qualities - what you get from the birds as far as egglaying and breeding performance goes is going to be a random assortment of the traits in the parent lines.
Can you explain this more? We use to get the Ross 308, I thought they were made by Aviagen. Though we don't do cornishX anymore, I have been thinking about getting a few and putting some of the ladies out with the free range flock. Is there a a strain of broiler that would be better for this?