I wouldn't talk about "Efke in general," since 25, 50 and 100 are really very different films. I don't think you can generalize from one to the other.

I also suspect you may be underexposing a bit, but it's like any film--do the normal tests for speed and development time, and you can figure out what works for you.

I've been using Efke and Adox PL100 in 4x5" interchangeably and haven't noticed any systematic differences. If you order them from different suppliers, you may notice differences due to age and storage conditions, but it starts out as the same film.