I’m not sure if this helps, but as someone who has experience with purebred ACDs, I just wanted to add that the “natural suspicion of strangers” the ACD breed standard calls for is very different from “stranger danger,” which fearful dogs of just about any breed can show. The ACD standard is referring to a certain coolly confident, calm, sharply scrutinizing “tough guy” stare that ACDs typically show when encountering strangers, which puts the stranger on notice without being dramatic. (By contrast most Border Collies show strangers polite reserve, without that air of sharpness.) But if instead a dog shows a tendency when encountering strangers to snap directly into a full-tilt pre-emptive threat display (nervous growling and tensing, reactive lunging and barking etc.), that’s “just” fearfulness, and can’t safely be assumed to have anything to do with any one underlying “natural” disposition towards strangers.
Because the working herding breeds tend to overlap so much in behavioral traits–extremely biddable and handler-oriented, great all-around natural athletes, obsessed with controlling movement, standoffish with strangers and needing lots of socialization in puppyhood to be comfortable around them–it can be really tough to pinpoint breed-specific behaviors in a herding mix of uncertain ancestry. Sometimes the tactics the dog favors during herding play can be a strong clue (e.g., dropping into a stalk and giving eye vs. darting in close to grip the heel), but even this isn’t foolproof, because none of these tactics are truly breed-specific, and besides a dog doesn’t necessarily need a lot of ancestry from any one breed to show its stereotypical herding behaviors.
Anecdotally BCs do have the reputation of being the most prone of all the herding breeds to fearfulness problems, but I’m not sure what actual veterinary behaviorist data might have to say about that, let alone the genetic factors that Darwin’s Ark is looking into.