This is coming from an outsiders perspective, a resident who only interviewed at MGH peds, so anyone feel free to correct me if I'm wrong...
Superficially it sounds like you already will be a great candidate. MGH peds seems to be on about the same level in terms of residency competitiveness as other NE programs like Yale, Brown, NYU, Mount Sinai, slightly less competitive than Cornell, DC Children's, John's Hopkins, much less competitive than BCH, Columbia, CHOP, and more competitive than tufts, umass, uconn, dartmouth, montefiore, cohen, rochester, st. chris, dupont.
If you want to make yourself as competitive as possible then get honors in your peds clerkship and peds sub-I. That is the most important factor when applying. Other factors, like medical school caliber, research, 'activities' and work experience, personal statement, letters of recommendation and performance on interview day matter, but only a little bit each individually . I'd imagine step scores are not a super important factor, although a higher score is always nice. If you already know you may want to go into a specific peds speciality and want to involve research in your career, then I'd recommend continuing to be involved in research during medical school for your career's sake, though it sounds like you already have done plenty for getting into MGH.
That said, there are many amazing peds residencies out there. You should keep an open mind, even going into the 4th year interview process, as you may surprised to realize what is available for training opportunities. If you are strongly interested in a career in academia, then doing residency at BCH (if possible) would be preferred over MGH, in terms of opening up doors post-residency.
Hope that helps. Keep in mind others may disagree from me on points, as some of this is just my opinion.
