I'm no expert in this but I'll tell you our experience.
My 12 y/o enjoyed doing the practice tests for the ACT, so we did quite a few and I spent quite a bit of effort preparing him. He did so well on the practice test I went a little overboard thinking I could coach him up above 30 in everything but math because he was in the high 20's his first try.
The difference between his first practice test prior to any coaching was within a couple of points of his actual score a few months later.
It seems that there were a few things we were able to fine-tune, but the score difference was very small. He stayed within the same few points on each subject on every pre-test, and carried that same general score on test day.
If I could do it again I would have spent the time doing something else. It seemed in our case there was pretty much a ceiling to what he could do maturity-wise, and that was that. The questions he'd miss were typically those that required inference or on subject matter he just didn't 'get' at age 12.
HTH
