Thanks for all that great info and insight! I don't really have a plan for these birds if they hatch, but I hope you're right about the cross - if I can get these birds to lay, I might just cross them to something else, to achieve a better sustainable meat bird. If not, we will just eat them.
I'd put money on it that the large hatchery is breeding for CX, not breeding CX to CX. I've raised only a few hundred so far and I've experimented with rationing and come to some hard conclusions. I think it's possible, but a lot more work than it's worth, to slow down an actual CX's growth. There have been lots of folks who have, over the years tried to find a more perfect, better balance between speed of growth and longevity and well being. It is not my experience that it it is worth trying to grow them out as pets or for eggs, they are a genetic wonder as a meat bird. there are many propriatary variations of the cross. the best one, IMHO, is the COBB 500, it stays pretty plucky, has a decent life and has a great grain to meat conversion rate, about 2.5 lbs of grain to a lb of meat. I use a sort of industrial grade mother heating pad to grow them out, no heat lamp, they are forced to sleep at night, so they take a natural break from eating. I grow them out in the fall, the days get shorter and nights longer, as they get older, which I find to be a great balance, I don't see leg problems and I get nice 7 to 9 lb birds at 9 weeks. I've concluded that they have something like 10X the metabolism of the typical heritage breed bird, that they produce so much of their own growth hormone that it's best to just give them the food they need and aim for a 9 week harvest. after 9 weeks they tend to get pretty burdened by their biology and I just don't see a way to give them an enjoyable life. there appetite is so intense that I think you could make an argument that restricting their diet too much actually leads to suffering. I try and find a balance, give them a good life with good health up to harvest time, at which point I kind of feel that they have reached their natural end due to their extreme genetics. I grow about 35-70 out a year, produce hundreds of lbs of organic meat, almost all the meat for my family of 4, it all gets vacuum sealed and frozen and I find the ability to show my kids a way to be somewhat self sufficient with growing our own meat in a human way, even in the suburbs and it feels kind of glorious, though the last few weeks can be trying.