Also, you will notice a sort of cut off point with each of the breeds or even strains. The older they get, the more you're feeding them and your relative yield goes way down as time passes. At some point it's not worth it anymore to pour more feed in a chicken, unless you have other reasons to do so besides putting them on the table. And then, of course, the older they get, the tougher to chew they will be and you will have to cook them appropriately.
For instance, when I started my little white broiler/brown layer hybrid experiment, I decided a 2kg (4.5 pounds) yield at 20 weeks would be ok. The bigger ones easily yielded that, the smaller ones was another story. I butchered some that only yielded around 600 grams (less than 1.5 pounds) at 20 weeks. Keep feeding them beyond 20 weeks is a waste to me. For the money I spend on feed beyond that to get them to gain a few grams I might as well get some caviar and eat that instead of chicken.
This year I had a surprise, though. I got fast growers that also put on meat very easily and the surplus of those that I decided not to use for breeding got butchered at 12 weeks. My family was very happy and thankful for the tender meat and said the next batches should be like this one.
I like my chicken meat with a bit more taste, a little redder and with a bit of a bite, so if this surprise repeats itself I will have to do with the roosters I used for breeding and the spent layers when the time is there. Or I can choose to let some slower growers run a little longer, without spending a fortune on feed, to satisfy my taste buds as well as my family's.
I don't think this is the case for you (your freezer is full you said in your opening post) but if the point is to put relatively healthy meat on the table economically, nothing beats buying CX chicks and raising them to the 6, 8 or even 10 weeks when they're ready to be processed. I've done so myself for years. The main reason for my custom broiler experiment is that at times, and for a variety of reasons, it's very hard to get your hands on CX chicks over here. The reason I used a brown laying hybrid is because the broiler pullet dropped dead at 4 months before she lay a single egg and we were again in a situation where the sale and transport of poultry was prohibited. In short, if it's still easy enough where you are and the economy of healthy chicken on the table is what you have in mind, get CX chicks.