Don't get me wrong a cao could easily kill a feral dog, or multiple feral dogs one after the other. Easily. The thing is to actually achieve this in real life scenarios a heightened prey-drive and a tonne of speed is necessary, or insane rabid feral dogs, because real feral dogs aren't going to push their luck, they're going to high-tail it as soon as they feel the risk outweighs the reward, which is fairly close to immediately after a cao even looks at them.

You can choose to listen to the stories of breeders with obvious clear agendas to hype up their breed, OR you can learn from the leading scientist that studied livestock guardians around the world. Ray Coppinger. He has written books and scientific peer reviewed articles on the subject and through his countless years of in-field analysis he has determined a full on fight between lgd and predator is exceedingly rare, and an lgd killing a predator even rarer. It's not a real part of the job, their job is to protect livestock from predators in the most practical way possible. The frankly infantile under-reasoned assumption is that the LGDs must have epic fights with predators and maybe even kill them, etc etc. This for starters would make no sense, for LGDs to force unnecessary fights on predators happy to move on, it will just result in them getting injured (win or lose) which will then subsequently negatively effect their ability to defend the livestock- the whole point of utmost importance, as opposed to kicking ass in fights with wild animals, which isn't important at all. And the prey drive to kill a large animal that's happy to run away and escape, that's a rare quality reserved for intense hunting dogs, the antithesis of livestock guardians in every respect. An lgd knows it's defending it's flock from a threat, it's focus isn't on determinedly killing the predator for fun. That attitude can only be found in a predatory hunting dog.

A bulldog/bullterrier type dog mentally will have that frame of mind to want to kill the feral dog and be determined to do it, but physically the dogs are just going to be too slow to ever catch a feral dog. They'll just be enthusiastically plodding along behind the feral dogs that will disappear into the wilderness. The only dogs that actually kill feral dogs regularly aren't lgds or bulldogs, they're sighthounds. You said your bull arab used to kill feral dogs, I can assure that doesn't come down to fighting ability or protectiveness it comes down to speed and prey drive. A dog with less of each (which includes all the dogs you're talking about) won't be catching any feral dogs. 
Which to me sounds like a good thing for what you're after, an lgd sounds perfect for you but don't expect to see any feral dogs get killed or even beaten up. They're not gonna get caught by an lgd or bandog or bulldog unless they through circumstance get cornered or caught by surprise or whatever. It could be considered a strange freak occurrence. I had a bandog catch and kill a fox once, but that's pretty unusual and happened because the fox found itself cornered.

I'm not saying a cao lacks the ability to kill feral dogs at all, it's ability is gross overkill in fact. But feral dogs flee, so the killing or fighting ability of the cao isn't the issue, it's the hunting ability to pursue and catch and kill a fleeing feral dog. Having the mentality and anatomical design to do that, it doesn't. Not that I see why this "shortcoming" would be a problem. Unless you want to rid the area of feral dogs, in which case get a staghound, even could keep it in a duo with a cao. Then you'd have all your bases covered.