I think that improvement is totally variable. I took Princeton Review and they said their average increase from the diag score was 10 points. Most people told me that the last diagnostic score tends to be fairly indicative of your score because, at least for Princeton Review, we used AAMC-issued MCATs. People tended to either go up or down about 1-2 points.
I agree that test companies tend to make their first diag test hard, and plus, you haven't studied at all by that point, so people tend to score fairly low; I did, as did my friends.
But just to illustrate how variable it is, my roommate took the test and she improved 8 points and went up 1 point from her last diag. I improved 15 points, and I went up 6 points from my last diagnostic score. And yes, I was completely caught off-guard by that sort of jump. I have no idea how that happened or why, but it just goes to show that you may get dozens of people's opinions (and believe me, I did), and it still may not tell you how you personally will do on the test.
In terms of studying, your study habits are your study habits. I took the Princeton Review class and devoted significant amounts of time to studying (which basically is what it took to finish all the homework that they assign you). But the one word of advice I have is *don't* study the day before the exam. If you haven't learned it yet, you won't in one day and the break from studying the MCAT was one of the best things I did. Do something relaxing to take your mind off the test. Be sure to get out of your house/apt, so you don't sit there thinking about it.
It's a rough test, but good luck with it!
