I ran some quick tests and discovered the problem, or rather, the issue. The Fulcrum's grandmastery only works for warrior classes, not priests, mages, or rogues. I tried giving the Fulcrum to each member of my party in turn. Sarevok and Minsc got the full bonuses, while Haer'Dalis, Aerie, and Jan did not. Oh well. I guess there's no way to break the class restrictions on proficiencies.