>However, there is very limited evidence that SAT coaching actually increases your SAT score. The NACAC did a study on this and they found that average gains of test-prep students is ~30 points (this was when the SAT was 2400 points).

IIRC SAT's equivalent of Matura in Poland, so I'll be talking about my case

I've been taking advanced math exam and I had some time + some money (like 10% of minimal wage) during winter break and I decided to buy 3 lessons on analytical geometry cuz I've been terrible at geometry, but since that was analytical, then I've seen a chance to get into that

I've attended those 3 lessons, did some exercises and guess what

on official exam there actually was a task from analytical geometry and I managed to do it and receive full points, which basically increased my score by 10 percentage points (that's a lot, I'd say)

Saying that 10% of minimal wage spent was equal to 10 perc. points is naive, but you get the point

What if I were attending those for whole year? 2? 3? hard to say.