My DS was 9y10 when he took the WISC and the WJ and I think his score was an underestimate. I had heard the >10 was a problem for HG kids because of ceilings so I wanted to do it before he turned 10. In retrospect, 9y10m was already too late. Even with the extended scoring, I think there just aren't very many questions at the high ends of the tests and it becomes easy to run out of questions. If a kid missed a few easy ones at the beginning, there is no way for them to make them up if the run out of questions at the high end. I think that is what happened with DS.
I think the ACT/SAT make a lot more sense. Every school knows what they mean and don't require the same amount of explanation and don't have the ceiling issues. Maybe your DS would like to take a practice test to see if it is fun.
