Yes they were developed from an original leghorn but the ones the factories use are developed strains for mass egg production in a short amount of time. 300 egg a cycle is not normal. You need improvements to get this. The strains used in egg farming do not adapt well even the ones that are never put in production. I have had them and they do not work out as a rule they are different is all. But they do lay well.
Leghorns (Pearl White/Production type) are not some scientifically developed test tube bird. Leghorns are an old heritage breed. The production layer was developed from regular old heritage Leghorns and they are no different from them in their ability to function outside of factory conditions. In fact, the flightiness of leghorns is intuitively something that as a factory farmer you would think that you would want not to have a problem with. Certainly the leghorns I had despised confinement even in my large open air pens as much as they disliked smaller pens, much of the nervous behavior I have seen seems exacerbated in confinement. I have had plenty of Leghorns, production type. I would add that in terms of flightiness, they are about the same as any other mediterranean class chicken in terms of behaviors and adaptability to free range, group confinement, etc in my personal experience. To me it seems that you are suggesting that production leghorns were developed for confinement systems and therefore happier/better off in them. If that is your point, I respectfully have to disagree. You just can't convince me that should these hens have more space to move their wings, etc. that they will be worse off or perform less well. If that were the case, i would suggest it is due to poor management of those systems, not the extra space, etc.