I realize physics questions are not a good fit for us, but considering the user posted their own answer, I don't see it as a problem. It was closed for being off topic, but looking through the help center, I don't see an actual reason this is off topic. Physics is actually a bit relevant when it comes to A Certain Magical Index, as it is a story where magic and science collide, so one could argue it falls under "Historical or societal context of an anime or manga".
Anime physics came at a point where we didn't know what we were doing and trying to find a niche. The question of why Mikoto's railgun is so destructive is frame more toward looking for an in-universe explanation to attempt to explain things drawing in realize physics as an analogy of sorts. Anime of course is not typically grounded or constrained by real-life laws of physics. Those types of questions had their ups and downs, but we can still have some fun sprinkling a bit of math on our answers.
The mentioned question in my opinion really straddles a fine line between something hypothetical and something unreasonably absurd. Such as aside from Level 6 Shift Mikoto being under the influence of Kihara Gensei. It would be our of character for Mikoto to launch a person as materiel for a railgun (it would be more of something Mugino or Accelerator [maybe Kakine] might do). She sticks to her arcade coins for a reason. Mikoto throughout Railgun and Index reaches different peaks and power spikes. Level 6 Shift Mikoto (mentioned to be 2% of the power of Level 6) and Mikoto's Liquid Proof (i.e., liquid coated) Railgun, supported by Misaki, piloting the Anti-Art Attachment are both extremely powerful, but on different levels and circumstances. Both can be considered "maximum potential". In these forms the power and potential of these forms really exceed the bounds to real-life physics, so how do you expect to calculate the uncalculateable?
As for the physics part of the question, there are a lot of variables to consider and a lot of assumptions to be made, the heat capacity/conductivity of a human for one can vary by climate, environment (indoor v outdoor vs in a vacuum), and what a person is wearing, their water, fat content, method of conducting/heat generation (microwaves vs sonic waves vs conductive current vs accumulated air friction by some means) etc. A person of Mikoto's build is likely to be below that of an average person (she is a middle schooler to high schooler, depending on which time period you look at, after all). Even if we attempted to infer optimal conditions, with in realistic bounds, what question about the series will this question really answer? The question itself is not unlike power comparison of characters in ideal scenarios with ideal power ups, sure we can indulge in power fantasies, but will questions like this be useful for anything but filling head canon? How fast can Mikoto heat up a burrito in average would be more reasonable than how fast can Mikoto burn up a human body at the very peak of her power.
Even if we were to focus on only the physics side and ignore the fantastical anime power level bits, it's more hypothetical thermodynamics than anime and manga related. Without the Railgun power level stuff it might be something better suited for Physics.SE, if they even take these types of questions.
I will of course abide by the site's consensus, but I would like to state my dissent as far as closing this question goes.
This question is similar and wasn't closed: How is Misaka's railgun so destructive?
I personally enjoy seeing these types of questions. They show a high degree of passion for analyzing a work and applying real world principles, and I believe that will draw in more visitors to the site.
As I've stated previously, I think the user showing their willingness to answer their own question makes a difference. And this user has given an extensive self-answer that might be of interest to others.
However, based on how we seem to have agreed overanalyzing is bad (Is it ok to overanalyze an anime?), it seems to me site rules are being upheld by closing this question. So, again, I'm simply stating my dissent.