I worked with students throughout the school year with a focus on the underlying content, and only switched to "coaching" the last few weeks before the exam. For many students, I tutored them weekly or biweekly, for 1-3 hours per week, for multiple years!
NONE of the studies on the effect of coaching consider the effect of this sort of longer-term individualized instruction.
I'm willing to believe that short-term coaching only has small effects, but sustained individual instruction has a huge impact on mathematical ability. And as I explicitly said in my original post, SAT/ACT do a good job of measuring that ability.
But claiming that sustained access to individualized high-quality teaching doesn't effect performance on subject-specific tests that require nontrivial content knowledge and practice is, on face, absurd. At the very least, the studies you're citing say absolutely nothing about this sort of sustained intervention.
(Also, College Board loves amplifying those studies. I wonder why...)