For me personally, I thought that the difficulty of the passages in Kaplan verbals were similar to the verbals I saw when I sat the test, however, the test day passages did seem a little longer, and a lot of other people reported the same thing. So if you are scoring double digits on kaplan tests that is probably a good sign, but see if you can do so without taking the whole time allotted (i.e. see if you can finish 4 or 5 minutes early) because you dont want that to hold you back on the real thing if the passages take longer to read.
That being said, having read a lot of sdn posts it seems like there are other sources with more difficult passages (Exam krackers and TPR I believe? someone with experience with these will hopefully comment) that you might want to try out so that you will be prepared for harder passages too in case you get a particularly tough section on the real thing
EDIT: forgot to answer your last question! Once I started hitting an 11 or 12 on my practice exam verbal section I found it pretty much impossible to score any higher. I think that is because the difference between a 12 and 13 or 13 and 14 is usually like one question, and it seems like every test has about 4 or 5 questions that have two really good answers/are somewhat ambiguous, and at that point it is probably just luck as to if you pick the answer they intend you to. However I have seen people score 13s and 14s so dont think its impossible with enough practice and perhaps a little luck!