I had a formerly feral horse as a teen, the local riding stable used to buy up horses off a native reserve outside Edmonton and then break them to do the dude string rides. Nowadays, a horse like that is called a “wildie” locally, not a mustang.
Horse was tough and surefooted and fast, but she was also a submissive temperament and when I got her, just another greenish but safe enough, head to tail, dude string horse.
I also got to see the fate of several horses that were clearly out of the same herd as my horse. Honestly by the time us kids were buying them, there was nothing about them that was different from any other grade horse (except some of them had really funky conformation).
As far as athletic, that’s going to be the luck of the draw. I would agree that natural selection is going to make them tough, and that constant low level exercise will make them sure footed and develop leg and muscle and good hooves, but not necessarily more than a technically “tame” horse that was raised on rangeland until they were 3 or 4. I do think both ranch raised and feral horses benefit from having more movement earlier on than a paddock or backyard raised foal.
But the things that make for economic and efficient movement in the wild do not necessarily translate to the kinds of “athletic” specialization that we ask for in performance horses.
A few of the isolated bands of horses have an Iberian look and carriage, but I’d want to observe them up close and in direct comparison to a good Andalusian or Lusitano to really say they are equivalent.
The less isolated bands are a real genetic mix, with regional variations. I haven’t seen unmanaged wildies/ mustangs in person, but have always been interested in them because of my first horse. My understanding is that the following characteristics are good survival traits but may not translate to performance horse athleticism: short, low strides; a lowset neck; a shorter neck; smaller overall size (minus a fairly recent influx of draft blood, most feral horse populations end up around the 14.2 hand height, and in more austere conditions like Sable Island or Chincoteague devolve into pony size).
These characteristics might make a really good solid old fashioned ranch working horse, adapated to the terrian it would be used on. But that’s different from making a great jumper, or dressage horse, or even a modern cutting or penning horse.
However, because of the genetic variation from region to region, you do need to look at the horse in front of you. I understand TB stallions were used in the West to breed remount cavalry horses up to the 1930s, so there could definitely be strong lines of TB blood in some mustang populations.
The BLM only passed the first wild horse management act at the end of 1971. Basically I would imagine that up to say the 1970s, when ranch horses started to disappear, that there would be a fairly fluid movement of horses back and forth from feral herds to “tame” and vice versa. Feral herds were a source of free using horses, but also I am sure many horses escaped or were turned out loose when no longer needed.
I too would say that a formerly feral horse that is being resold should cost comparable to a grade horse of similar training and conformation.
