Earlier quoted context omitted.
Intelligence needed to be existentially threatening to all human life <<<<<<< intelligence necessary to survey the totality of atoms and particles in the universe. The former is still very possible.
LessWrongers seem to fear a superintelligence that was 95%+ effective at persuading/manipulating humans to achieve whatever its ends were. But this would require the AI being able to predict the future to a degree that wouldn't be remotely possible given the computational power available to it, due to exponentially increasing difficulty.
Given we seem to have a decent range of persuasiveness even amongst these very very similar minds, why do you think the upper limit for persuasiveness is a charismatic human?
Though even if that WAS the limit I'd still be somewhat worried due to the possibility of that persuasiveness being used at far greater scale than a human could do...