I think a good LOI will make a difference. I am not sure if it will make a BIG difference. However, the letter must be good itself with lots of supporting statements as to why you like the school (specific details about the curriculum, students, etc). Plus, I would assume that it would also be beneficial if your LOI could somehow set you apart from the rest of the LOI's. and of course I would assume you will need to somehow assert your true enthusiasm for the school. Best of luck....!!