As the second study suggested, 33% is optimistic, but 2C warming based on 450ppm seems overly pessimistic. The article you referenced isn't peer reviewed and is written by a high school teacher. He seems to reference an article though that appears well written, but asserts that the global warming will be catastrophic and that a tax on carbon is the only way to stop it. Are these people scientists or politicians? They should stick with the data and let the policy makers worry about the policies, if and when they're needed.
Around 2C warming for a doubling of Co2.
And this one suggests it is around .51C for a doubling:
I can provide many more below 2C estimates if you'd like, as climate Co2 sensitivity is where I've been doing most of my reading.