As a parent of a child who hit many ceilings, my understanding is as follows: The extended norms are helpful if a child is close to the ceiling of the test (FSIQ 160) and you want to know how far above that ceiling the child actually scored on GAI. They are also helpful if your child is particularly high in one area (hits more than two ceilings), as they help provide a better understanding of the overall IQ in relation to that area of giftedness. Unfortunately, the extended norms do not necessarily provide a true picture of the EG/PG child, and many testers recommend using SB-LM if you hit several ceilings on WISC-IV and have to use extend norms. The Gifted Development Center has a nice explanation of scoring the WISC-IV and the SB-LM on its website.
