Maybe it's not about sheer power. Maybe it's about the fact their sorcery creates no Mark. I think Kellhus has some need for Meppa, otherwise why not just shred him with the metagnosis? Why leave him alive? He's important for some reason. But that begs the question, why did he exterminate the Cishaurim in the first place if they held some kind of importance to his own goals? Could be Meppa was a plant, either by Moenghus (I can hear Wilshire groaning already) or by Kellhus himself. If he was 'programmed' to behave in a certain way, to give Fanayal false hope of toppling the New Empire, that may have been worth it just to bring Fanayal out of hiding. He could very well be a Dunyain 'agent' of sorts. Would explain the whole 'he doesn't know who he is' comment (whelming and all that). If he was Moe's agent (stop it, Wilshire) it could have been as a retributive strike against Kellhus (not very likely, to be sure), or as a piece to the whole metaphysical predicament of Earwa. Something Moe figured out that maybe he thought Kel would need.