MKSAP is the best source of IM questions I have ever seen. I did very well on my IM shelf with using those as the sole question source. Definately don't worry about a percentage that you get right. it shouldn't be used like that. I probably only got 50% right (maybe not even that), but i wasn't keeping track. read the questions and read the explanations. plain and simple. learn, learn, learn from them. they are not meant to be an evaluative tool.
most people i know hardly got any right and they all did great on the NBME shelf.
tough questions, great step 2 prep. i was getting about 50% avg, maybe a bit more. but, they were great as a resource, really hit high yeild material hard and in depth. if you understand all of that book, the IM on step 2 will be much easier.