DS tested on the WISC at 9y10 and hit serious ceilings and his score was probably an underestimate. We then waited for a couple years for him to be mature enough to sit for the ACT and SAT before we had the information we were looking for.
So for HG+ kids I sort of feel like there is a gap between 9 and 11/12 where there isn't much great testing since the SAT and ACT require a level of stamina that may not be reasonable to expect from a 9 or 10 yo. Given our experience, I guess I would suggest getting something around 8 because you don't really know what you might need through those gap years.
