It is not simply about how much research you do, but about who the person is doing it. You could be much more prepared to enter a PhD program than people with much more experience than you... it is up to the program to determine whether or not they will accept you. Don't let me dissuade you from pursuing your goals, but it is less than a sure-thing of getting into an MD/PhD program from an MD program for anyone. If someone is less qualified than their normal applicants (based on research, academic record, knowledge/experiences in research, and future goals a person desires a PhD to accomplish), then that person is less likely to be admitted.
Again, it is school-dependent and situation-dependent. Some schools will have no slots, others will have slots for qualified students. Joining the program as an MD student is not a sure thing for anyone. For instance, a few years ago at Emory, 3 students applied internally for 1 spot, while around 9 spots were reserved for traditional applicants. Your best bet is to apply from the outset. The internal track is not a sure-thing, so you should be aware that if you matriculate to an MD program, depending on the structure of the program and their history of taking internal applicants, your chance of entering the program from this route is not great.