I just want to add in that all of BPshinners advice is great, and he should definitely be listened to. Sorry I've been a bit slow, I've been very busy running around trying to get a house, helping out with various travel things and so on.
All right, back to the argument. Our definition of ceilings is at issue. My definition relies on the fact that it is a general ceiling, for as many people as possible, while yours relies on the individual having a ceiling. Since OP asked whether 'everyone has a ceiling on the LSAT', I decided to pursue the path that it was a general ceiling that applied to most people.
Obviously, not everyone has the same ceiling, since different people have different levels of comprehension and skills for this test, so I had to choose a ceiling that affected the most people possible to effectively answer his question. That's how I came up with reading comprehension as a ceiling. As a note, not everyone has this problem, but this is the one factor I think that would significantly limit performance on the test. For example, if English is your second language, you are at a disadvantage when taking this test versus someone who has read it their entire life. If your reading and writing skills languished in college, you will be at a disadvantage on this test.
Reading comprehension is not as easy to pick up as critical thinking, analysis, organization etc. It takes approximately 9 years to teach you to read effectively in the first place, and I know several adults that still read at a middle school level. These are the people I refer to as having ceilings for the LSAT. It will take them years to catch their reading up to where they can comfortably read and answer the RC sections.
I hope this clarifies my argument. I do not think that it resembles your definition of a plateau. It is a serious problem that these people would have to REALLY push, and it could take them years, depending on their situation. This to me, is a ceiling.
Ah, OK, I've got you know (I think). You're trying to find something that defines what the ceiling is going to be for people in general, while I'm creating a floating point that is a maximum potential score for an individual.
Now that I've got that down, I think we completely agree, with one caveat. I think my critical thinking and your reading comprehension are the same thing - the ability to analyze what you've read instead of just regurgitating it.
I was trying to apply that to say that it will define a maximum score that a certain person can get, while you were generalizing it to state what will define that number for most people.