Just like dogs there's good breeders and bad breeders. I find most of the folks that show or breed to the SOP for large fowl do also pay attention to the production qualities and eat culls. Bantam breeders not as much. One thing about the "form follows function" as much as I say it and believe it, I think the truth of it is
"
form ALLOWS function(!!!!!!!!!!!!!!!)".
Birds bred to the SOP have the correct body type to match their original purpose and excel at it. However it is possible that those traits have not been selected for. However if you get a start in birds with the proper bodies/confirmation/type YOU can select for production qualities and have an easier time of it then if you had started with birds lacking in those qualities.
One thing to remember is to be realistic, "Heritage" breeds are no longer the forefront of production, for eggs it's expendable hybrids that produce heavy at the cost of longevity and health. Meat is dominated by terminal crosses that are lucky to survive the 7-10 weeks to butcher. It wasn't until the 1950's that dual purpose fowl started producing over 150+ eggs a year consistently, 16 weeks was the absolute minimum to attain a market weight.
As far as the right chickens, it depends greatly on what your exact purpose is: Eggs and very little meat, Eggs with a good chunk of meat, Meat with a good chunk of eggs, or meat with the occasional egg.
For the first, the streamlined continental and mediterranean class white egg layers (Leghorn, Ancona, Spanish, Andalusian, Buttercup, Polish, Hamburg, Campine, Lakenvelder etc) will do a fantastic job,
For the second (Eggs with good meat) something like a Rhode Island Red, Langshan (the best breed), Welsummer, Dominique fits well.
For the third (eggs/meat with more of a meat emphasis) Wyandotte, Dorking, Orpington, Jersey Giant, and the like.
For the fourth, primarily meat with a few eggs, Cornish, Brahma, Delaware are in their element here.
There's some overlap and opinions, but that just kind of gives some ideas.