Everything you've been 'hearing' is probably being told to you by someone who has never had a 'real' cornish, and I bet doesn't know anyone who has either. The birds you're buying from hatcheries, typically don't have anything in common with a real bird that's close to the SOP, much more than color- and lot of times the color is WAY off.
Cornish are great birds for the things they bring to the table, namely meat. The strain that at least 3 of us on this board raise will smoke any other line, breed, composite, or strain of fowl out their for being the 'best' backyard sustainable meat bird, I'll put money on that.. This bird strain is superior at being broody and taking care of those babies. I have 2 Silkies running around here for the sole intention of hatching eggs, and while on average the silkies will brood 3x a year, and most Cornish only 2x- I have a few momma hens that'll embarrass the SIlkies.
Now, if you asked me to put my birds in an egg laying contest, I'd have to decline. About 5 eggs a week from April to Aug is about all their good for. I bet that's at least an egg or an egg and a half more than the strictly show ring oriented Cornish will put out. In addition, it doesn't seem to matter whether I let pen 1 free range or not, as they'll only get about 30' from the coop.