FWIW, being severe, I had an immuno absorption and 0.5 mg/kg IVIG. Obviously I can't tell which of these (if any) caused the effects.
I think it improved my PEM. After the whole endeavor I crashed for only 3 days (expecting multiple weeks). The PEM also felt less "bodily".
Also my heart rate got noticeably lower and my muscles stopped hurting for about 4 weeks.
What was left was debilitating fatigue and unchanged cognitive symptoms. Much like
@ryanc97 said. Actually, I even felt more fatiqued than before, but I prefer it to being completely wired all the time.
Overall, it did not help or change my quality of life (or Bell Score) much, but it also did not hurt. Probably just a waste of money, but I felt that I had to try.