To my knowledge, most talent searches are based on grade. Typically, they explain this by noting that grade-advanced students will have had instruction in higher levels than their non-grade-skipped age-peers. There is something to be said for this placement rule, even though it feels like there is a penalty for having programmatic and cognitive access to above-age-level instruction, because it does seem a bit inequitable to compare the math achievement of someone who has not had the opportunity to receive instruction in algebra to someone who has, even if they are of equivalent learning ability.
Some might find above-grade-level testing to be valuable beyond access to supports/programming because it may provide more finely-grained measures of a student's true instructional level. (Above grade level --> how far above grade level) Along those lines, some of us use them as progress-monitoring tools, or documentation for homeschooling portfolios. It's achievement testing; it doesn't measure the same qualities as a WISC does, so outside of the access function, it doesn't provide the same kind of data as cognitive assessment does.
