I doubt he got less than perfect grades in med school because of not being smart enough...it was probably that he didn't get along with people that well. Med school grades are very subjective, particularly during 3rd/4th years.
I don't know that there is a large enough sample size of 4th year med students who bailed out of a residency match to do "private equity" - or any other one particular job - to be able to give you any statistics on your odds of success. You think you will succeed doing this private equity thing, and you may very well be right. I am thinking you have a degree from some Ivy League university like Harvard and you did very well in undergrad. Given that, I would think that even if the private equity thing didn't work out, you would be able to get some other banking or finance job. It might not pay that well, but might very well pay as well as fm/peds/psych/physical med/rehab or whatever residency you'd be falling back on in your situation.
People in your situation are so rare that I really don't think you are going to find someone on the internet who was in a similar situation to the one you are in right now. Not only are you not going to get any hard data/statistics, but I don't think you are even going to get a "case report" of someone in a similar situation. I attended a so-called "top 5" med school and there was one person in the class ahead of me who bailed for a job in business consulting (not sure which firm). This was before the Match and I know that she didn't even pursue matching into anything because she had decided medicine was not for her. She did want to finish her degree just for the sake of safety and having something to fall back on. I think many of us do know people who have bailed out of medicine for some type of business job after doing a residency plus/minus fellowship (sometimes right after, and sometimes after practicing for a few years). Most people who go to med school are either dedicated to the idea of taking care of patients and/or are risk averse enough and/or have enough student loan debt that they would not entertain the idea of taking such a risky job and throwing away a ROAD spot.
Is there any reason that you would do your residency except the money? I mean, do you like seeing patients at all? If you hate it, then you're going to hate hate hate residency...and would hate practicing as an attending probably even more. I'm about to accept an attending job in cardiology and I will have 4 days/week of seeing clinic patients - unless doing interventional cards most people only get 1 day/week of doing procedures like cardiac cath, etc. I'm talking about seeing probably 30+ patients/day perhaps. That's a lot of damn medicine to be doing for someone who doesn't seem to like the practice of medicine very much.