Have we not been able to breed cows and cats and dogs and horses for all sorts of things and traits many SDs beyond their ancestral wild populations?

Not for intelligence, as far as I know. Though dog breeds are widely considered to vary in intelligence -- have there been any attempts to quantify it?

As to "failing", traditional genetic engineering certainly ran into some limits. To continue with dogs, large breeds have shorter lifespans. Many breeds have well-known pervasive genetic problems (hip dysplasia in German shepherds, etc.).

It sounds like your beliefs on this topic are molded by some outdated Romantic myths about genius.

I doubt it's Romantic myths since people that come to my mind mostly lived in the XX century, but yes, I've said that it's a prior and I'm open to evidence other than handwaving.

since no one knows what other genetic changes one would make

That's the point of experimenting :-)