My 14 y/o took the SAT in 7th and the ACT in 8th. We found it somewhat useful but not a major thing. The benefits were giving her practice with the tests before they mattered, that she got an award from the talent search (which she liked) for her scores, the scores would have qualified her for summer programs through any of the talent searches had we had the $ to spend on them, and that her high school was highly impressed with the scores b/c her ACT scores from age 12 beat out the average score of their high school juniors so it might have helped with advocacy there. On the other hand, her placement in high school classes was in no way contingent on the scores and she already had her GT id.
I did want to comment on the "achievement/ability" thing, though. The SAT used to be an aptitude test, which correlated with ability fairly well whereas the ACT has always been an achievement test. When the SAT was renormed maybe 10 yrs or more ago, it was changed to be more in line with the ACT and is no longer considered a quasi ability measure. However, both tests can reasonably be considered aptitude tests moreso than achievement tests when used as above level measures especially if the child is not prepped ahead of time. By this, I mean that you don't go and sign him up for the Princeton Review ahead of time not that you don't give him any idea of what the test will look like or how he should approach it (answering questions he knows first and skipping harder ones, etc.).