Well, according to Kaplan, it's not at all weighted. And it's not performace based like the GRE where if you get a question right, then the next one is harder. There's a pool of questions pre-selected before you take the exam and 50 questions are randomly selected for each block, so it may appear that the test is getting harder or easier at certain times, but it's still random, not systematic.
He also said that the popular belief about scores being influenced by who takes the exam at the same time as you is a myth. Your score is your score regardless of how well or how poorly anyone in your cohort performs.
....admittedly he also couldn't tell us an actual "maximum" score though....