My very limited understanding of sadomasochistic relationships is that they are built on a high degree of trust and consent. Thus, the masochist consents to a certain experience and power relationship.
Rape by definition occurs without consent, and therefore would be outside any such consensual relationship. People all vary in how they experience traumatic events. Nonetheless, I would not expect someone who engages in consensual masochistic behaviour to find non-consensual rape any less traumatic than someone who is not masochistic.