I scored in the 99th percentile and I didn't take a psychology course at all, ever. I only studied what was in the Altius study materials. If you look at the exam contents from a purely statistical perspective, I think there is no question that a second semester of biochemistry is worth *much* more on the MCAT than advanced physiology, psychology, and certainly neuro. Biochem is heavily represented on two of the four sections!
By the AAMC's own definition, and my experience, the MCAT is designed from stem to stern to be a CONCEPTUAL examination of basic science. All my experiences would support that. In fact, some of my friends who were taking advanced science courses for their degrees would often get tripped up because they would over-complicate simple questions by bringing in "Well, we learned in my Advanced Vertebrate Physiology 490 course that the dorsal root ganglian *actually* do not function in exactly that way (i.e., in the way intro courses teach...and the way the MCAT is testing).