I find that with HP5 in Rodinal, either 1/50 or 1/25 it loses a lot of speed, it is only with Ilford film I have this problem, using Kodak or Foma I lose about 1/3 of a stop, I don't know why, but with Ilford I only get around 320 even in D76, that goes for all Ilford film, Quality is fine but it seems to lose a lot for me, but it is a few years now since I have used any Ilford, prefering Kodak Tri x or Fomapan 400
Richard