How many physician-scientists pursue academic tracks?
I have no idea how you'd find hard data on the program itself and its results, but as an idle musing I checked how many full time faculty members in the Johns Hopkins School of Public Health's Dept. of Epidemiology held an MD/PhD or DrPH. The department was chosen as a very good department in a very good school with a strong medical school that I'm not affiliated with.

Ten of the 94 listed faculty members were MD/PhD or DrPH's. Little under 11% of the faculty. And that's not including other possible "physician scientists" like MD/MPH degree holders...if you do that the number rises to 23 faculty members with an MD degree in a related, but non-clinical research department. Nearly a quarter of the faculty total.

Of course, this is only a very crude proxy for how many physician-scientists pursue academic tracks, and even the representation of clinician-scientists in research departments will likely vary wildly by said department. That being said, I've met a considerable number of them in my graduate school career, either entirely in academic settings, or balancing research with practice. It's absolutely a viable path, though not an easy one.

As another data point, here is the list of alumni for the UNC School of Medicine's MD/PhD program: http://www.med.unc.edu/mdphd/fps/alumni-1 . That should give you a decent glimpse at where those particular graduates go - it looks like a fair number ended up in research or hybrid positions.