I follow this thread because there are so many knowledgeable folks on it. Me, I'm a newbie with a mixed flock of hatchery birds. I'll be getting my Heritage Ancona's this spring and will line breed them as described by the late Bob Blosls.
My comment is in regards to egg production or some lines lack thereof. I recently finished "High Egg Production By individual Hens, Pens and Flocks," It was originally published in 1922. I downloaded it free from Goggle books. Everyone used trap nesting to increase egg production. There's a thread for it here
https://www.backyardchickens.com/t/567357/the-trap-nesting-thread/440 at Backyard Chickens other than the thread I never hear mention of trap nesting; Old timers swore by it.
How the heck can one be totally completely sure how many eggs a hen lays without either trap nesting or keeping hens in solitary pens? You can't. A quote from the book:
"The Hogan system, as we understand it, has not worked out well with us. Have studied it and tested it but did not find it reliable. With us that plan failed badly, and both ways. Birds that by the test should have done well, did not; while others that ought not have laid well, proved to be good layers. One hen that laid 249 eggs did not show up well by the Hogan test, while another that did extra well laid only 179 eggs.
When a man uses trap nest for FIFETEEN YEARS (my emphasis) -uses them every day in the year, keeps careful records and studies these records=he has a lot of theories knocked out of him. His judgment is the 365th day."
After a few decades of trap nesting egg yields where astonishing, with beautiful prize-winning SOP heritage birds even! It's no surprise people who remember farm flocks from the 50's envy the egg production of those birds.
Many breeds can come close to laying as well as leghorns when managed properly. I'll tell you from the book many breeds were laying almost equally well, Leghorns are used today due to feed to egg conversion, It's simple economics!
I'm going to trap nest my Anconas, I have the luxury of being father to six home-schooled kids, hey if I have to feed them they can at least check trap nest for me. Trap nesting with good record keeping will yield results it's a fact.
Another thing as anyone who understands genetics and just the basic laws of mother nature knows: everything will return to it's natural mediocrity quickly without proper oversight. If no attention is paid to egg production It is not long before egg production will drop off. The author states that many farm flocks that pay no attention to production have 60 egg per hen flock averages.
Bottom line: No reason one cannot breed beautiful SOP Heritage birds and increase production at the same time.