Let me try to clarify a few things from an M1 perspective. BMS students take biochem, cell bio, physiology, and neuroscience with us. The curves are based on the medical students' grades. We don't have any classes which are curved down, so if you get a 70/80/90, you're pretty much guaranteed at least a C/B/A even if the class average is high.
Of course it's hard to get an A, but just in the sense that you have to work hard, not in the sense that other people's grades lower yours. Most of our test averages have been around 80 for the M1's, which is what the curve is generally set at anyways, so there are plenty of people getting A's.
Former BMS students who have been accepted into CMS are called M1A's, although I'm not sure if that's an official designation or not. They have to take extra classes in one of four fields, such as public health, nutrition, etc, since they have taken most of the M1 classes last year. The main classes they take with us are gross anatomy, histology, clinical reasoning, and embryology (I think). I assume that they tend to have higher test scores because they don't need to study for the other subjects (biochem, physio, etc) on the integrated exams. Or maybe they just spend less time studying, or are busy with their extra classes/research. In any case, it doesn't affect anyone's grade, because anatomy is not curved.