the curve on the MCAT does not depend on how well people do. it is only based on the mix of questions. i think each question or passage has a difficulty level assigned to it, and they reach into the hat and pull out a mix of passages and discretes which will add up in difficulty to give a particular bell curve in the results. that's probably why they stick "experimental" questions into the test, so they can figure out the difficulty score to assign to the question. so it's like they're field-testing the question.
https://www.aamc.org/students/applying/mcat/preparing/85436/preparing_understandingscores.html
i think the practice aamc tests are very close to the real thing. the real test might be slightly harder. but i don't think by much, maybe 10%. it's not as hard as the kaplan FL tests. so the (predetermined) curve might be a bit more lenient than the practice tests, but not as much as the kaplan ones.
on the practice aamc tests i always had about 12 minutes time remaining at the end, but on the real PS section i scrambled to finish and didn't get to really work on the last two questions. it was partly due to it being early in the morning, and partly due to my overconfidence and letting the clock run down too much. i should've paced myself faster. i don't think it was because the real test was significantly harder than the practice. others may feel differently though...
anyway, i hope the curve is a bit more lenient so i can hit my practice average.