I believe that non-named Psykers typically are limited to Lv 2. Rabble with PM Lv 3 would be, hypothetically, as Psychically powerful as Mephiston. And more powerful than any non-named Space Marine Psyker, to the best of my knowledge.
Eldar, GK, and Daemons are probably an exception, but Lv 2 is usually the top out, I think. Even GK squads are only BoP lv 1, I think.