What are we asking?
Production methods must be considered. Many factors contribute to performance outcomes in "broiler" production. Yes, Breed is a primary contributor but so is the breed's suitability to the methods of the fowl keeper.
One's methodology is where I find much differing of opinions on breeds and their "performance" virtues and vices. As methods differ so follows differences in opinion. Another difference is simply philosophy in keeping fowl. Penned fowl or Ranged fowl? Grain bins and barns or Grass-fed and pastures. This squabbling is centuries old so with that in mind allow me to offer up why Delawares performs as well as today as a Modern Industrial Broiler (MIB) called the cornish cross.
The MIB is a breed bred for penned rearing model with its higher grain inputs. Read as feed grain very cheap but production space relatively expensive. Where as the "heritage" broiler was a bird historically expected to forage on pastures for the majority of its diet with minimal grain inputs. Read as feed grain relatively expensive but space very cheap.
If we are talking "backyard" birds for meat and we are agreeing to a "Yard" or pasture (not runs, tractors, or growing barns) as integral to the production of said meat.
Now if one is unaware of broiler production on pastures, I'm referring to the esteemed Bresse fowl keeper practices.
https://www.backyardchickens.com/threads/french-standards-for-raising-bresse-meat-chickens.866299/
http://www.farmerseekingroots.com/bresse-poultry-in-the-south-east-of-france/
The model and methods are sound and the principles can be applied to even the small "backyard" meat producer. In this model Delawares have two key performance advantages over the MIB, that being long legs and temperment. Foraging requires drive and mobility. MIB aren't know for there good legs or feet. MIB are also bred to be docile because over crowding causes neurotic behaviors like cannibalism. Delawares get an advantage here as the MIB is generally less active and thereby lacks in supplementing its daily feed intake by foraging.
To the carcass quality question. If a 5 lb cleaned carcass at -10 weeks is "quality" well I guess Delawares and every other breed of chicken beside the MIB are not "quality". If you feel quality is how big the breasts the animal produced were, here to the MIB is the only breed of "quality". On the other hand if you feel the "quality" broiler carcass is more about taste than portion, sustainable practices more profit margins, animal welfare and breeding for structural soundness more than shorter production cycles; then Delawares are still competitive as a broiler breed.
A few things to keep in mind about Delawares:
Breast meat production is not the focus of Delawares it is in the leg quarters.
Delawares reproduce independent of human assistance.
Delawares are a true breed.
Delawares were the broiler industry gold standard prior to the introduction of the MIB.
Apples and oranges folks.
