I was not satisfied at all with the hatchery type Cornish I once tried, though have seen others here that seemed to be a bit closer to true Cornish in type which means they have a more meaty body along with coming a bit closer to the Standard of Perfection. The body type that good Cornish have is simply not designed for heavy egg production.
I've never tracked feed to meat ratios, so can't answer that question. My hatchery type were good at ranging, but being more active is not necessarily a good thing for maximum weight gain or feed conversion, and they were slow growers as are quality Cornish; but good Cornish pack on a great deal of meat and make a dressed carcass that looks very much like a commercial broiler in shape. While the true Cornish are slow to mature, they are more meaty than any breed I've known, and to me much better flavored than the fast growers. They aren't a noisy breed, though the cockerels/cocks can certainly be heard when they do crow and the pullets/hens do have a song when they lay. Both the hatchery type and quality Cornish I've had lay medium sized eggs, the hatchery type were better layers. Quality Cornish are not huge, but will provide more meat faster than the the giant breeds like Brahmas or Jersey Giants. However, I would not recommend Cornish as a dual purpose flock for anyone wanting to sell excess eggs; most people wanting to buy fresh country eggs want large eggs. Those bred towards the standard of perfection are the ultimate meat breed in my opinion, with heavy breast meat that seems much better flavored than commercial white meat along with short, heavy thighs and drums; and do lay enough eggs in season to keep the flock going plus provide extras for eating. I keep handful of other hens for large eggs and winter eggs, but consider well bred Cornish a great self-sustaining meat breed. With their hard feathering, that body shape you see in the pictures of my birds is the same shape as the dressed bird.