The truth of the matter is its 90% an individual effort, and I suspect the same person would do about the same on Step 1 whether they went to the #1 ranked school or the #150 ranked school. Med school gives you a bit of foundation and structure for sure, and makes your board preparation easier to the extent the material isn't totally new, but for the most part all schools cover the msjor topics, and people threafter prepare for the boards learning from the same First Aid and qbank/World questions. About half of all med students at places without mandatory attendance opt to skip the lectures and learn on their own anyhow, so I don't think you can credit the med schools with their success or failure. And the lower ranked schools often get accused of "teaching to the boards" while the higher ranked places supposedly spend more time on loftier details (ie things that won't show up on tests but are interesting and cutting edge of interest to academics) so to the extent there is going to be built in board prep it's probably inversely related to US News rankings. I think the amount of time the school gives you to study before the boards may be relevant to how well you are able to prepare, and some people do need the egging on of much smarter cohorts to stay focused, so tougher admissions might be useful for some just to give some degree of fear/ competition. But I'd still say that for the most part this is a very individual effort, and you will do well or poorly on the boards based on the kind of effort you put in, not based on where you attend. (Fwiw I don't think there's a great correlation between shelf exams and the steps.)
Programs dont publish their board scores precisely because they want to remain flexible with different kinds of curricula (PBL, etc), and don't want to be regarded poorly as compared to the program that gives students 5 months off to study, hires a Kaplan course, and "teaches to the boards". It's presumed in academia that you can water down med school to a mere board review course and probably score as well or better, but in the end they fear programs that do this will crank out very mediocre doctors. So there's a tacit understanding not to publish board scores, and whatever numbers you see floating around on SDN and elsewhere are self serving and are mostly bogus.
There are things that can set schools apart, and make one better for you thn another, but I think board prep is more of a confounder because odds are you will skip a good hunk of your lectures and study on your own with the same board resources and do more or less exactly the same wherever you go, assuming you put the same effort in. The only thing that can screw you up is if you are happier in one setting versus another because that translates to your focus. I would probably focus more on things like quality of 3rd year rotations as paramount in your decision process, if you can get the info.