I have tried to research the economic benefits of cognition enhancement, and they are quite possibly substantial. But I think Roko is right about the wider political ramifications.

One relevant reference may be: H. Rindermann, Relevance of Education and Intelligence for the Political Development of Nations: Democracy, Rule of Law and Political Liberty, Intelligence, v36 n4 p306-322 Jul-Aug 2008 argues (using cross-lagged data) that education and cognitive ability has bigger positive effects on democracy, rule of law and political liberty than GDP. There are of course plenty of reciprocal factors.

As I argued below in my comment on consensus-formation, in many situations a slightly larger group of smart people might matter. The effect might be limited under certain circumstances (e.g. the existence of big enough non-truth seeking biased groups, like the pro-ethanol groups), but intelligence is something that acts across most of life - it will not just affect political behaviour but economic, social and cultural behaviour. That means that it will have multiple chances to affect society.

Would it actually reduce existential risks? I do not know. But given correlations between long-term orientation, cooperation and intelligence, it seems likely that it might help not just to discover risks, but also in ameliorating them. It might be that other noncognitive factors like fearfulness or some innate discounting rate are more powerful. But intelligence can also co-opt noncognitive factors (e.g. a clever advertising campaign exploiting knowledge of cognitive biases to produce a desirable behavior).