I read on here a lot about people with breeding programs geared toward maximizing carcass traits. Most of my chicken experience has been in raising pullets to point of lay and selling them in town where people can't raise straight run and have roosters or can't have but so many, excluding most minimum orders from hatcheries. I generally butchered the roosters myself or sold them at a discount for meat. I will share some knowledge I have gained from being in a cow calf beef operation for several decades.
Hybrid vigor is a real thing, it's the closest thing you will get to free pounds. There are limitations to this. If you cross breed A and breed B, there is hybrid vigor to be had. The more dissimilar and unrelated those breeds are, the more hybrid vigor is available. In other words, there is probably more hybrid vigor to be had in a Shamo Orpington cross than in a White Rock Barred Rock cross. There is such a thing as maternal hybrid vigor. This means that, say a brood cow, will have hybrid vigor that will give her a boost, she will be more hardy and produce more milk for the same feed input and this will help her offspring. She can be a cross of two of what are considered maternal breeds and then she can be crossed on a "terminal" breed known for carcass traits, (if care is taken to make sure the terminal sire doesn't throw offspring too large for her to have.) This might not have much impact on the chickens size, as chickens don't give milk, although most game breeders that are looking for size use two year old hens to make sure the egg is going to be nice and big and throw big chicks to give them a little head start. Where maternal hybrid vigor might come into play is more eggs to set, all other things being equal, a hybrid hen should be slightly more productive as she should be more hardy.
Beyond a three way cross, there are actually studies on cattle showing that hybrid vigor starts to be less evident. Also, when using composite breeds and crossing onto breeds that are components of that composite. At some point hybrid vigor can turn into "mutt regression" it would seem. Consider if you are using hatchery birds that are, let's say hatchery RIR and crossing them on hatchery BO that are just hatchery RIR with buff color added. There will be less on the table than if using a pure line of each from a breeder. There is a certain amount of hybrid vigor to be had from crossing two pure inbred lines of the same breed. If you are making an 8 way cross, there might not be as much hybrid vigor to be had compared to making two 4 way crosses and linebreeding them and selecting them to aworking standard of uniformity, and then crossing those two lines to produce your meat birds. Or maybe just sticking to a two way cross.
Likewise, when making your composite breed, don't guage your success until the hybrid vigor wave has run out several generations into your breeding program. If breed A and breed B are crossed the offspring should be bigger than either parent, it happened because of hybrid vigor and not because of your brilliance in combining those two or three lines. When you get several generations in, this size boost COULD disappear. If you cross a big breed with a small breed, you can select the biggest offspring with the desired traits of the smaller parent and breed them back to each other until those traits are stabilized, but that is selective breeding.
It seems the active goal of many is to have a self sustaining flock that produces fast growing roosters, The benefits of hybrid vigor can be lost in trying to produce one line of birds with all of the production traits you want. It might be better to buy pure hens and a different breed rooster, and once you have cross bred hens, buy a different rooster of a third breed and then sell off the three way cross hens and start new with pure hens. I know this goes against the idea of in house self sustainability, but something to ponder.
Just thought I would add some things for prospective backyard producers to mull over.
