You also open a can of worms here! …because this discussion is narrowly confined to physical pain only.
Who is to say that the psychological, emotional or mental pain inflicted by some other process is not experienced more intensely by, or in fact more damaging to, the person experiencing it?
As an on-topic analogy, who can say that isolating a pupil into solitary ISS confinement is not a more intensely painful psychological experience for that person than a short sharp shock would otherwise have been?
Just as you’ve eloquently outlined above, we simply cannot know this. Only the person who experiences it can know it, and hence the often alluded situation within this forum where SCP was chosen by the pupil themself over the alternative offered. !