I have a 34 MCAT which I'm debating on whether or not to retake. A 3.6 science GPA and 3.65 cGPA. I hope to improve my sGPA to a 3.7 which I think is entirely possible. I attend a top 20 undergrad that has a top 10 medschool attached. My biggest extracurricular has been work study in lab. My sophomore year I was just a lab tech but the summer after I started doing real research, presenter a poster, and will be published by the end of junior year. Other than that my EC's have been interning at a clinic doing office work (with patient interaction).
So, from Neuronix's sticky:
Applicants who did not receive an offer: GPA 3.68 (SD .24)/MCAT 32.7 (3.7)
Applicants who received one offer: GPA 3.74 (SD .21)/MCAT 34 (3.7)
Applicants who received more than one offer: GPA 3.82 (SD .15)/MCAT 35.7 (2.5)
Basically, a 3.7 / 34 MCAT means you're likely to receive an offer. The difference between a 3.6 and a 3.7 is hard to say. I'd say if you can pull it up without killing your other ECs, go for it. If it's going to mean taking a lot of time away from the lab, might not be worth it.
As far as undergrad prestige...it might benefit you in terms of who you know (say, your PI is on an admissions committee or has friends) but is unlikely to help you in terms of GPA, at least that's the consensus I've heard (and gathered from perusing MDApps). Perhaps if you're a chemical engineering major at MIT or Caltech or something, you'll get some help, but a bio / chemistry / science major is more or less just as difficult at a state school as at a top 20 (at least in my experience...I'm at a top-20 and have a friend in the same major at state school...we've compared notes).
MSTP's are by virtue more competitive than partially/non-funded MD/PhD programs, and some of the programs you listed are the -most- competitive.
In my opinion you aren't completely out of the running at the schools you listed, but it would be very foolish to apply to just schools of those caliber. This isn't meant as an insult to you (your stats are pretty good), but it's just a fact of the matter that NO ONE is a sure thing at any of those schools MSTP's...a small number of spots and a lot of applicants with 3.9-4.0 and 36+ MCATs are applying. You can stand out in other ways, of course, but you have to be pragmatic, too.