I could consistently help even the poorest students move from below 50th percentile to 75th percentile. Moving from < 600 to mid 700s is totally doable with sufficient tutoring [1]. Even for pretty dumb students.

I think SAT/ACT are pretty good tests [1], but they're horrendously over-gamed at this point. I have very little faith in either as anything other than a demonstration of how badly the student wants to be admitted to a good school and has money for tutoring.

[1] edit: i.e., SAT/ACT are not easy to game wit short-term coaching, but sustained tutoring can substantially increase students' performance... see thread below for further elaboration and discussion of "coaching doesn't help" studies.