Corellation != causation.
I'd say the exhaustion of Europe in WWI and WWII (which the US only participated with its military, whereas the European citizenry had much more direct costs across the whole population (occupation, bombings, ethnic cleansing, brain drain, etc), the post-war influx of European scientists (half of the scientific developments in post-WWII US are from Jewish, German, and other immigrants), the collapse of European colonialism, and an internal 300+ million strong market to try stuff in, plus a strong military to force its market interests globally, were more of a factor in that.
Case in point, China is becoming "the superpower it is" without following the same model as the US of "unchecked capitalism", if anything the opposite.
>I feel the US is the closest example in the modern world to capitalism unchecked - with both the pros and cons that entails.
What pros? If anything, unchecked capitalism tends to also destroy the free market and democracy -- as unchecked capitalists gain political and social power over their puny fellow citizens and lesser businesses.