Test prep is effective at getting you an accurate score.

One bit of knowledge improved my ACT score from 27 to a 33: internalizing how long I have for the tests. I was so used to being able to finish school tests with a comfortable amount of time remaining, so I took the test at a casual pace which means I didn't finish. The second time, I sprinted through the test as fast as possible and my score improved dramatically.

I doubt I could have done better with more coaching or prep.

Similarly, a single 30-minute lesson taught me what I was doing wrong on the ACT Science section (trying to understand the science instead of treating it as 'find the answer in the text'). Brought my overall score from 32 to 35 (I was already a good test taker, but those 3 points made a big difference),