If your aim is a 'top twenty' school then the extra year of research would be very helpful, as greater than two years of experience should be your goal (with a pub if possible).
Also: Aim for a 35+ on the MCAT. I'd suggest doubling the planned shadowing, and try to work in some nonmedical/noncampus community service as well (starting soon), ideally moving into a leadership position with time to further enhance your application's appeal. If a teaching/tutoring/TAing opportunity comes along, take it.