Do you have the raw scores so you can see yourself if the # is likely to change? That might give you a clue as to how much the overall # is an underestimation (if it is). My one child for whom we were told the composite # was a bare minimum and who has always appeared more than the MG-mildly HG her FSIQ and GAI appeared on the WISC at 7, had a big jump when I went back and looked at her one 19, which lent credence to my assumption that she was more able that the test indicated.
Like a pp said, it is more likely that the ceiling is "hard" when there are a lot of scores in that 18-19 range, but when you're dealing with just one 19 that jumps to something like a 22 or 23 with extended norms, it might be a good clue that the other #s in that index might be a bit low if they aren't similarly high. I hope that I am making sense here (very tired!).
For instance, my one whose 19 would have jumped to a 23 had an 8 on block design, another test in the same index. She wasn't given any alternate tests in that index and, from what we understand, refused to complete the block design test so the big jump on the 19 on one of those other two tests in PRI was a reasonable clue to us that the PRI was a low ball based not only on the 19 moving, but that she presumably could have done better on the the one where she got an 8 had she not frozen on it.
