>>> The average purpose bred horse (about 90% of the total population) is no different in ability from the average non-purpose bred horse of similar build.
Well right there I see two important stipulations-- "average", and "of similar build". If you breed with a purpose--
Lets say you breed for hunters-- and you get an average set and shaped neck, average legs, average size, average shoulder and hip angles-- put all that together, and then you get an average "build". Not the best, not the worst-- proabably functional for your purpose but not the biggest star.
If a "non-purpose" bred horse happens to be born with a similarly average "build", why yes, that horse should have the potential to have similar athleticism and movement....
But to me the caveat is that many breeders who are "purpose breeding" are usually breeding for ABOVE average horses... and most breeders NOT breeding with a purpose are NOT, they might get lucky, but the consistency will not be there. IMO those breeding with a purpose have more of a chance of producing a few average horses, more above-average horses, a very occasional flop, and an occasional star. Those NOT breeding with a purpose will have a greater potential of producing many average horses, a few flops, very few above average, and hardly ever a star. IMO.