Failing an SMP is a very expensive way to find out (or confirm) that you
shouldn't go to med school. If you can't succeed in an SMP you
won't succeed in med school. Keep in mind that 55% of med school applicants get rejected every year and
most of the rejections have pretty good stats. Med school interviews (fallibly) weed out well-qualified students who aren't mature/focused/independent/resourceful/personable enough to do the job.
That said, the only anecdote I have is that "I've heard of" a student who did multiple SMP attempts (I heard 3) until the student succeeded, and then got into med school. There must be a large trust fund in that story, as well as highly-questionable SMP admissions offices. Also it was nice to find out that there's a more hellbent and ridiculous GPA comeback than mine.
Doing average in an SMP is more scary to me than failing one. If you fail, that's very clear. Even when you're successful, there's a stunning level of misery in keeping up with med school coursework. A reasonable person would find a less misery-prone path, after
failing on the misery-prone path.
But if you do
average in an SMP, then you didn't really do anything to help your med school app. Typically, the case is that the student didn't get into MD school with acceptable-range stats, and they don't want to do DO or Carib. So they do an SMP. By doing average in the SMP, they aren't going to fare any better in getting into an MD school (certainly not in the worst case of California). So they end up having spent $50k for nothing - they could have gotten in DO or Carib in the first place.
Best of luck to you.