You have to take this with caution and a lot of caveats, but I think this peer-survey-based ranking from US News is potentially interesting information on this subject:
https://www.usnews.com/best-colleges/rankings/national-universities/undergraduate-teaching
As explained in the Methodology page:
In the spring and summer of 2023, U.S. News & World Report once again asked top academics to name the schools they believe have faculty with an unusually strong commitment to undergraduate teaching.
The rankings for Best Undergraduate Teaching, as part of the 2024 Best Colleges rankings, focus on schools whose faculty and administrators are committed to teaching undergraduate students in a high-quality manner. College presidents, provosts and admissions deans who participated in the annual U.S. News peer assessment survey were asked to nominate up to 15 schools in their Best Colleges ranking category that have strength in undergraduate teaching.
The Best Undergraduate Teaching rankings are based solely on the responses to this separate section of the 2023 peer assessment survey.
Then to sort of test your hypothesis (although the focus here is on teaching generally and not research specifically):
Brown is at #3, Princeton #4, and Dartmouth #5. The only colleges above are Elon (looks like around 89% undergrad) at #1, and Georgia State (looks like around 87%) at #2.
Then Stanford is in a tie at #24, Harvard in a bigger tie at #48.
This is not exactly proof, indeed I have not done anything else to try to detect a correlation between percent undergrad and these survey results (which I think would have to be controlled for other things to really be meaningful).
Still, kinda interesting to me your measure seemed to “predict” the different groupings of survey results (give or take) for those particular colleges.
Edit: Oh, by the way, in my view #24 or #48 are still good scores. Because there are way, way more colleges than that.
So I again want to stress that even taken at face value, this is not suggesting Stanford or Harvard would be bad choices. But it might–might–be another piece of evidence that if one of your priorities is “faculty with an unusually strong commitment to undergraduate teaching”, a place like Brown might be a particularly good choice, even among other still very good choices.