So I know that, on the basis of the much lower PSI, you would expect the GAI to be at least a little bit higher than the FSIQ, but the GAI also removes the WMI subtest included in the FSIQ, which is his highest score, resulting in a slight drop moving from FSIQ to GAI.
(BTW, the drastic difference between the two WMI subtests also parallels the verbal>>visual reasoning preference.)
IOW, no, if this FSIQ is not already qualifying, the GAI would not qualify your child for GT services in most districts, as it is actually lower than the FSIQ. If you were looking for a qualifying score, the one to pursue would be the VCI or VECI, as he clearly is strongest in verbal cognition (the VCI is the only Index in the Extremely High range).
On your second, and more important, question, yes, the PSI is statistically significant, and might be clinically significant. But you would need more information about his actual performance IRL (i.e., not just in the testing office) to determine clinical significance. The low PSI may or may not reflect his diagnosed ADHD (that is, it might reflect a separate processing speed weakness). And even if it does reflect his ADHD, it doesn't necessarily mean that the ADHD is inadequately controlled in terms of actual function.
