Ask Johns Hopkins to provide you with statistics of how many people get into med school from Hopkins and what the avg GPA and GPA range is. My school even provided a list that broke down the number of applicants to a particular med school, the number accepted, the number matriculated and for those students that matriculated (if more than 3 did) it provided a GPA avg. and range. From my school (Stanford), I saw that students were accepted to top med schools (UCSF, UCLA, Stanford, Columbia, Harvard) at a higher admission rate and a lower GPA avg. than the school's total avg. For example, UCSF accepted 20% of all Stanford students that applied with an average science GPA of 3.60 (range 3.2--4.0). That year the overall admission rate to UCSF was 5% with an avg. science GPA (of matriculants) of 3.78. I think this may indicate that being from Stanford was helpful for students applying to UCSF. This trend was common for many top schools. (As a side, Stanford had a lower than the overal acceptence rate to places like BU and Mt. Sinai)
However this finding is confounded by the fact that 1) a Stanford student will more likely be a good standardized test taker for the MCAT since they most likely did very well on the SAT and 2) Stanford and other top undergraduate schools provide students with increased access to unique and interesting opportunities than a large state school would (example: a friend go a 3K non-competitive grant to study travel to France to study French films; I got 3K one summer to live on campus and do research at the med center).
From what I have been told at UCSF (I know more about this school than others, hence why I am using it as an example), once you make a certain academic cut-off, your academic profile because a much smaller component of your review. They know you can handle the work, now they want to know why you will make a good student at their school and ultimately a good physician, researcher, etc.
At UCSF, my interviewer said that they do closed file interviews to eliminate some of the bias that an interviewer may impose on their judgement about a student regarding academics. She said that she would definitely be more interested in someone who went to Yale than someone who came from a state school. Without thinking about it before she met the applicant, those would be her first thoughts. I was surprised that she gave me such a truthful answer (I asked her why they did closed file interviews since we put so much time into our applications and I thought an interview should build off of that), but it is true that most people will be impressed with that kind of school almost regardless of how you did by a few GPA points.
Hopefully, that answers some questions and gives some objective evidence as to how your undergrad school may affect admissions. That being said, I don't know if I would put JHU in that category for undergrad. I am not making a claim about JHU being a good school or not (I think it is very good), but I don't think it has the reputation as being the top of the top. It won't have the same "wow" factor as a student walking in from Harvard. If it was a choice of Yale (etc.) vs Rutgers, I would say Yale hands down...
