While I have not personally shot any of the Efke PL 100 film, I did have the opportunity to read densities on some Efke negatives that Jim Shanesy (C6H6O3) sent to me. The film does build density very well for alternative process and Azo. I believe that Jim had rated the film at less then the mfg speed (possibly 50). The low densities of these negatives indicated that the film was over exposed at the speed that Jim had rated the film.
The film that Jim sent to me had been developed in ABC Pyro and also Pyrocat. I could make a case that the developer that you are using is not as active as what we use when developing negatives to a higher DR. Therefore it may very well be that EI 50 is appropriate for the developer that you are using. (As one develops to a higher DR the film speed actually will show an effective increase.)
Metering and Zone placement are equally as important as the EI that one assigns to a film. I don't know what you are using as a meter and how involved you are in exposure calculations.
I have ordered some 4X5 Efke and should have more pertinant personal experience a little later.