I had one melleri, out of 4 CBB babies that returned to health after suffering Vitamin A deficiency and I believe it was the use of performed A that made that possible.
This was 9-10 years ago, when many people advised against using preformed A and the baby melleri were raised on prey gutloaded with sweet potatoes and carrots, among other fresh produce. It took the melleri 4-6 months to show overall weakness, eyes being crusted shut and having secretions in them, sometimes the pupils became irregular and had adulterated margins. Vets were at a loss as to the cause. This problem was never seen in WC animals, that undoubtedly had stores of A, left over from life in the wild. I have never experienced any eye issues or oversupplementation issues using preformed A routinely. I find it works better as a prevention than a cure, because once signs of deficiency are evident, multiple nutrient relationships can be affected and multiple body systems and functions.