at our school we had the option to take the physio shelf after finishing 2nd year as kind of an indicator for where we stood in terms of preparedness for usmle. i can definitely see how it would be much easier after taking courses such as pathology and intro to clinical medicine. for those of you who will be taking it in the future i think it would be worth your while to take a peak at topics like congestive heart failure, COPD, some of the valvular stenoses/regurgitations, etc. for a lot of those things, knowing about the pathology will give you the answer about what happens to preload, afterload, etc, rather than having to think it out based on your physio knowledge (don't get me wrong. i know that's the point of the exam, but still...).
i get the impression all of the shelf exams go a bit beyond the boundaries of the subject itself - for path, it's definitely worthwhile to go over some biochem, genetics and microbio/immuno. best of luck to everyone.