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I've written an essay criticizing the claim that computational complexity means a Singularity is impossible because of bad asymptotics: http://www.gwern.net/Complexity%20vs%20AI
Still reading minor nitpick: for point 2 you don't want to say NP (since P is in NP). It is the NP-hard problems that people would say can't be solved but for small instances (which as you point out is not a reasonable assumption).