To reiterate what Dottie posted first;
Doing well on the CogAt likely indicates that your child is some level of gifted AND a good test taker. I don�t think it is uncommon for gifted kids to do less well than expected on group tests as many gifted people don�t like forced choices, among various possible reasons.
The Stanford-Binet or WISC are the real IQ tests as Dottie mentioned. It seems reasonable to follow the talent search model for achievement testing (give top 5-10% above level test) and kick it up a notch by using a cognitive assessment with more ceiling such as individual IQ test.
To save yourself possible grief and money, you may want to find out what/who the school accepts if you plan to share results.
