TLDR: Modafinil does something for my DP.
I don't intend to write up a detailed review about modafinil, but I will say a couple of things.
So, I recently had the opportunity to meet a couple fellow DP-ers from this forum (wonderful people), and one of them happened to have prescription MODIODAL pills on them, he was prescribed the substance by a doctor. I do not condone my following action, don't do this but I'm stubborn so I asked if I could give it a try. (you guys are aware how experimental treating DPDR can be, and psychiatrists in my vicinity do not know much about this condition, and laws here are pretty strict, not much leeway in what they can prescribe you). But anyways, he said sure, he doesn't intend to use it any longer so he gave me whatever he had left, so 8 pills in total (pls don't take drugs from strangers tho).
Disclaimer: I do not condone the use of illegal or legal substances that were not perscribed, this is very stupid. That being said I have done a lot of research into this compound, I will link the relevant studies below for information purposes. I informed myself as to the appropriate dosing, the less-well-understood mechanism of action, pharmacology and followed the necessary safety precautions.
Anyways, I gave it a shot, first carefully upping my dose starting at 50mg the first day. I did this to test whether or not I was allergic to the substance, or whether or not I would develop a rash from it (one that could be fatal). After day 1 I felt nothing, no effect whatsover from 50mg, positive or negative. So I decided to up my dose, at 100mg I still felt nothing.
It was when I tried 150mg that I could notice something happen to me. I sat down to read a book, and the first thing I noticed ~1 hour after taking 150mg, is a profound sense of euphoria, felt like a come up. Soon after I started feeling very on edge, everything began to look more sharp, lazer focussed feeling. I was a bit overwhelmed initially, but tried to keep my calm. It wasn't a bad feeling per se, most definitely tolerable, but I just experienced a shift in my state of consciousness that I was not expecting. The day went on, and it passed by really fast actually. Whatever I put my mind to, I felt more focussed and engaged. Things like reading were more engaging, and getting work done didn't feel like a huge chore. I stayed on this 150mg dose for a couple of days, and I haven't been as productive as I was on modafinil since getting a DPD diagnosis. Also, the jittery anxious feeling didn't happen anymore after my first dose of 150mg.
I had 250mg left yesterday, and I decided to take it all in one go. I took it on an empty stomach as usual, and as I expected, in about an hour I could feel the effects, and like the first time, a profound sense of euphoria hit me. I was super motivated all of a sudden and so started doing my work. The energy this thing gives you is fascinating, the focus you get from it is truly astounding, and a mental clarity is also felt. I did however stay up until 3 am last night, but this is because I woke up late, I took my dose at 9:30. It was as if I drank 15 cups of coffee that day, but without the physical jittery side effect.
Conclusion: what does modafinil do to my DP?
The main benefit of modafanil for me is increased concentration and focus. It is hard to assess this one, but I did feel that my working memory was better as well. I don't usually like to think of my DP state as one related to brain fog, but if I have any brain fog at all, then the modafinil definitely seems to help with that.
The aforementioned effects are pretty much expected, this is what the drug is supposed to do and does do in even healthy patients, but modafinil seems to affect my DP in some incomprehensible sense, it is difficult to put into words. It is not a cure for me, it doesn't fix my sense of unreality, but it does do something to it. Visually things look more sharp, I had a better sense of spacial awareness and things of that nature. That being said, I was not as self-conscious and self-reflective while on this drug, so I barely noticed my DP at all, especially on 250mg. I only saw the task that was ahead of me, my mind didn't wonder off at all to check in on how I feel.
Interestingly, I also felt more negative emotions during my experience, especially at the higher dose––which is inherently a good sign if you ask me. I do believe that behind the layer of my DP I am emotionally exhausted and so these negative emotions were sort of surfacing during my experience. For example, I felt embarrassed in front of some people I was talking to yesterday, and I could feel the shame/shyness in my body––something that I do not sense at all in the midst of my DP.
In terms of side effects, I didn't feel any, though I did lose all interest in sex. This is likely just because I was so immersed in other activities.
One thing to keep in mind is that I took the modafinil intermittently, not continuously one day after the other. It was more like whenever I felt like it, or when I felt that I woke up early enough. If I woke up after 9am I would be reluctant to take it, because this thing has a long half-life, it has its effect on me for 10-15 hours depending on the dose. True enough, the 250mg kept me up reasonably late. Moreover I only had 8 pills at my disposal, so I cannot say how this thing would affect me long-term, I do think though that this is a drug that is to my benefit, and it is a pill I would take if I was prescribed it. In the studies where modafinil helped with DP symptoms, it was used in a long-term window [1, 2].
The research is scare, there is no large studies testing modafinil's effect on DPDR, in fact, there are no published trials, only reviews [1, 2]. As such, we are left with mostly anecdotes, and my testimony likewise fits into this category.
There is a scarcity of studies concerning modafinil in general. There is some indication that it affects mechanisms in the frontal lobe and the hippocampus that are involved in reinforcement learning (norepinephrine and dopamine are somehow involved, yet the substance is said to have a fairly distinct mechanism of action from other stimulant drugs such as amphetamines like methylphenidate  and we know that when used, patients with depression make decisions more efficiently and effectively .
Based on the research and my personal experience I would say that if you are struggling with attentional issues or are hypo-aroused, then this could do something for you, but don't take my word for it, of course, always consult a professional about whether or not this is something you could/should be taking.
One last question that remained in me was why did I feel closer to my emotions on a cognition-enhancing substance? There was a study recently published that said in relation to modafinil that "cognitive enhancing effects in the absence of effects on affective processing suggests a promising potential to enhance cognitive control in clinical populations" [4, underlining added]. So somehow, it is presumed that the substance affects prefrontal regions in relation to emotion regulation, and as suggested by the DPDR literature this condition is very much related to over-regulation of emotions (overactivity of prefrontal regions).