I disagre with that thory, becxause of simple logic reasons.

Mirror neurons cannot work in that way simply because none of us actually experienced anything of that on our body. The could only do that in the situation when you actually experienced that event yourself and if it was strong enough to be remembered.

So the only thing our empathy can rely on is facial expressions and context. It does not matter if it is real or fantasy as when I see someone being hurt in the normal movie it makes me sad but if this is happening in guro manga it is enjoyable and if this is happening in the tom and jerry cartoon it is just funny.
Thus our mirror neurons are supposed to evaluate emotional state of another being and not just respond to whast is happening to them but evaluate lots of things including the fact if we actually care about that person. As if we like someone we will feel bad if something bad happens to them but if we hate them we feel happy when our enemies get hurt.

As for my own situation, like someone who learned about injuries and torture from movies, I see them as only mild discomfort.
In fact, I do not really care about pain even in real life as I see it not as suffering but as the annoyance which consumes your attention and as loss of functionality.
If I see someone being castrated all I think is "now he won't be able to have sex"

However in situations when I see some character suffering like she is crying or looks sad or something bad happens It is also stressful for me as well.
As It happened in one manga where the father was torturing/raping his wife along with his little son, and she slipped from the ropes fell down on her head and died and this was very sad for me too. Not because she died but because the boy was in shock as his mother just died as well as her husband who just lost his wife.
Same thing happened with "Mai chan daily life" manga where none of those horrible tortures had any efefct, but when may cry on 2 occasions when people whom she loved died it made me feel bad too.

Considering penis torture If I imagine that happening to me I in the fantasy also see it as only mild nuisance as it will grow back next day lol
but if I think about is as about real thing my response is that will just fer a month and then I imagine whast problems it will going to cause but whast happens at the moment is not very relevant.