Hi there,
I fear I don't have any information for you either. But I have always thought it was funny that the WISC-IV information article about extended norms lists the percentiles for the extended norm IQs and GAIs and every single one of them just says >99.9 percent. You do have a feeling that if there was a reason to do that extended norming, there should be a reason to put more information behind the result. I.e., GAI of XXX is 99.9xxx percentile. But I haven't seen it!
Hopefully wiser folks will have a more useful answer for you. But I do think it's a perfectly reasonable thing to ask. I suspect that tester just hasn't seen that sort of result, and didn't even know how to answer you. Not nice of her to get snippy with you, though.
Sue
