Having just emerged from the ocean of testing...I would suggest that reading through both WISC tech reports #4 and #7 would prove helpful to you.
It seems to me that GAI (and even extended scoring) is most helpful when you suspect the child has a learning challenge. FSIQ is supposed to be a composite score of four different categories. Together, that score is useful in determining strengths and weaknesses. My sons' strength (VCI -152) balanced out his weaknesses (PSI-126) to give me a composite number that matched up quite well with his achievement test scores.
Here is a quote from the WISC tech #4 that I found helpful during the testing process:
"It is important for practitioners to recognize that the GAI is not necessarily a more valid estimate of overall cognitive ability than the FSIQ. Working memory and processing speed are vital to the comprehensive evaluation of cognitive ability, and
excluding these abilities from the evaluation can be misleading. The classroom performance of two children with the same GAI score but very different WMI/PSI scores will likely be quite different. In educational situations where evidence of a
significant AAD is required to obtain services, the GAI may be used as the ability score; however, the WMI and PSI should still be reported and interpreted.'
HTH
-cc
