How were you able to regulate feed while raising them with other breeds? I know that for most chickens people give free access to food. How would you do that without the CX eating it all and getting big?
This has been addressed but I'll expand on it some. To me this is a qualified myth, not necessarily correct in all conditions. I'm talking about the dual purpose chickens more than the Cornish X.
For thousands of years people have raised chickens without feeding them anything during good weather months. There are some qualifications. You have to have good forage. Many farms do. They have pasturelands, hayfields, orchards, wood lots, and even having other large livestock around gives them a lot of food resources. You can't do this in a suburban backyard lot, you need a lot of area with good forage. You have to be able to free range them. If predators ae too bad you can't do this. You'll notice the OP has some that sleep in trees. Can you do that with your predators or risk tolerance? Depending in winter weather you may need to supplement their feed. Poop management is a lot easier if they are spread out all over instead of confined to a smaller space.
You are not going to raise grand prize winners at a show, those pretty much require a special diet. Regardless of breed, they won't grow as big or as fast as they will if you free feed them, that's sort of the purpose of this thread. Their eggs won't be as big as they would be if you feed them a higher amount of protein. You can still eat the chickens and they will lay a lot of good eggs, but neither the birds or eggs will be as big as they could be. With them free ranging it's harder to make them the kind of pets that will perch on your head and shoulders or climb into your lap for a cuddle. This training generally involves handling and food, hard to do when they are free ranging and you are restricting what you feed them,
The commercial meat industry and commercial egg industry have developed birds that provide a lot of meat and eggs very efficiently if the birds are managed a certain way. You are not going to get the same results if you manage them differently. I consider overfeeding them to be the biggest medical risk to Cornish X and the commercial laying hybrids. They are bred to use food efficiently, either to make meat or to make eggs.
To me, if you can raise and keep chickens without buying much if any of their feed that's tremendously efficient way to feed your family. That will not work on a commercial basis, commercial requires too many chickens if that is how you are going to make money for your family and lifestyle.