I found that my practice scores were dead on to my real scores. I never took a full timed practice exam, but rather did a full timed section and then reviewed it right after. So I would take a section like PS in the morning, and then in the afternoon study PS with particular emphasis on the questions I missed. I would get a "10-11" on each section every time I took one, and I got a 32 on the real thing (10,11,R,11). I actually thought I would have done better on the actual thing since on the practice exams as I got to the end of the section I would start getting edgy knowing that in a few minutes I would know how good or bad I did, while on the exam, I felt more relaxed knowing that there was nothing I could do but drink beer and eat wings after the exam.
I don't know if it was just me, but I always found there was one passage that always screwed me in each section. Like for most of the passages, I would miss at most 1-2 questions, but there would one passage where I would get 8 to 10 of the questions wrong, and it always pissed me off since it was always this one damned passage that was keeping me from hitting the 12s and 13s I thought I could reach. Ah well...