Although there was a study, it has since been discovered that the participants were all from wealthy families, with children who attended private education and who were expected to be above normal intelligence anyway, due to their background and inherited genes. CAH alone cannot make patients more intelligent, in fact it has been associated in a  minority of cases as causing learning difficulties, especially in salt losing boys who are not immediately identified and have suffered severe dehydration of the brain during infancy due to a crisis.   I have 3 children, only one of whom has CAH and I would say my affected son is on a equal par intellectually with his brother and sister.