Fred's Hens :
I'll report on 4 breeds (strains)
Black Sex Link (commercial RIRxBR) Good layer, but at 6 lbs, bred to be an average eater. This is a plus, if cold hardiness is the goal, but feed to egg ratio quite average, nothing special. Excellent forager which helps only part of the year in northern Michigan.
Myers Golden Buff (production red) Superb layer, but slightly above average appetite and feed consumption.
Spotted Sussex outstanding forager, fairly light eater when able to forage. Heavy body. Mediocre egg laying.
I.S.A. Brown the feed conversion winner by a long, long way. Laying machine, very light body, and requires less feed than any breed/strain I've ever kept. Pretty aggressive forager, which is surprising, perhaps. None better at feed conversion to eggs laid.
Hope that helps.
It might be interesting to note that feed conversion and foraging capacity are not the same thing. Industrial crossbreds, which are by definition not breeds insofar as they do not breed true, have been selected over time for excellent feed conversion, but their husbandry plan does not incorporate in its scheme the ability or need to forage. They are developed for a closed system in which they consume commercially prepared grains only. For these birds, a conversation of feed conversion is necessary because the bottom line will be how many eggs exit for how much feed enters. They come from industry for the purposes of industry.
Of the birds you listed, the only breed, which is a long standing heritage breed, is the Speckled Sussex. Their very existence predates the manufacturing of grain; thuds foraging is a key element of their particular heritage.
The Dorking is an excellent forager, but this makes sense. If we consider it particular history, it has existed for so long without the benefit of scientifically formulated diets that foraging is distinctly in it blood. Old English Games, Mediterranean breeds, foundational Dutch and Belgian breeds, these birds long predate the feeding methodologies upon which we rely today.
Having said that, foraging is going to be strain related, and it is something for which on must select. This is at the very heart of landrace breeding. Of course, it presupposes that the homesatead possess adequate forage. With this given, you would raise up your stock on forage with little more than a hand out in the evening. At six months, you go over your stock looking for signs of vigor, thrift, and fleshing. Use your Standard to check for structure disqualification. These go on to be your breeders. You continue this process yearly, and gradually your birds will show, via their development, that they have adapted to your terrain.
Of course, the presupposes that you are beginning a sustained breeding program because otherwise the fruit of your selection will be lost in outcrossing.
In truth, there's more than a little bit of randomness in comparing fowl purchased from hatcheries, by which I am not slamming hatcheries, simply making a statement about their growing practices and those things that they expect their stock to do from one generation to another. I would place a fair sum of money that foraging is not among the vast majorit of hatcheries breeding goals. Without continued selection for thrift on forage, this capacity will start to dwindle. Thus, it's probably safe to assume that any particular hatchery stocks ability to forage is a vestige of where those birds were when their bloodlines were first procured by the hatchery. Moreover, given all of the variables that are possible between hatcheries and the stock they possess, it's safe to say that Orpingtons from this hatchery will not have retained the same foraging ability as Orpingtons from that hatchery. If one considers the essential nature of breeds, it probably safe to say that Silver Spangled Hamburgs, even from hatcheries, are going to forage better than Brahmas from hatcheries--due of course to the dum luck of history. Also, that any particular breed offered by a hatchery forages well is not really going to be indicative of a general quality of that hatchery but rather, once again, of the sheer luck of history.
If foraging is truly high on your priority list, and you want to see that capacity increase annually, consider choosing a breed that has, since its inception, been asked to forage; Dorkings, La Fleche, Old English Games, Hamburgs, Redcaps, Campines, Lakenvelders, Minorcas, Leghorns, Anconas, Dominiques, Javas, etc.... Then, make 'em go to it.
Good Luck~