I've also been taking kaplan and aamc full lengths, and surprisingly they have been rather accurate. I suspect it is because they are generally more accurate towards the upper end because even with Kaplan's curve, there needs to be substantial analysis and reasoning in order to get a 13 - 15. But once you get to 10-12s on Kaplan, it doesn't matter if you get all the hard questions wrong, you can still get that score with the curve.
another difference is you have to be absolutely on top of all other questions to get a high score on AAMC, because there is no room for error. for kaplan, you can get away with silly errors b/c the gap is very generous.
having said that, the reason the kaplan scores are not entirely meaningless is because they have scaled the test to include more difficult questions, with the assumption that you will perform per difficulty level on the AAMC tests. so if you do as well on each difficulty level question, then you should obtain similar scores on the AAMCs.