These welps are growing like weeds. Ive reduced their feed to 16% layer feed to attempt to slow their growth rate. Maybe not recommended but I'm going to have to start butchering early. They should only be around 6lbs. Even the small ones are bigger than 6lbs.
The extra calcium in layer feed is of **long term** concern for the kidneys of non-laying chickens, particularly roosters, and more so in developing weeks pre-maturity. As your roos are not expecting a long-term relationship with the living (well, maybe THEY are, but you have other plans...),I'd judge it of much lesser concern than your Cornish eating themselves into morbidity by way of liver damage or similar.
NICE weights, btw!