And an another important point is that Graendal's fame came largely from her non OP skills. We knew she treated the minds the OP couldn't touch, and we don't know if the code of ethics of the AS let her use some forms of compulsion benevolently or if this was forbidden and she developed those skills, helped by her psychiatry background, only once a Forsaken.

Rhavin is largely described as a politician/diplomat. It's quite possible he never shone, despite his strength, as a channeler (the same can be said of Balthamel, for instance, and Moghedien). He surely was well trained, and surely he had to lend his Talents to tasks required by the Guild like everybody else, but it doesn't sound like he made a "OP"-focused career, the way Semirhage, Aginor, Lanfear or even Ishamael (as philosopher of the Wheel/Pattern/metaphysician) did. Demandred, like LTT, seems a bit of an unknown.. obviously both were very skilled channelers and it sounds likely they owed their ascension in the Guild to the merit of their service as AS, but as for the details we'll never know.