Dagong, the Chinese rating agency claims that US GDP is under $6 trillion once you take out the fake financial padding out of the US GDP. To quote them:
"In addition, due to the high economic financialization, more than half of the profits in the real economy come from the returns of financial activities. If we exclude the factor of virtual economy, the U.S. actual GDP is about 5 trillion U.S. dollars in 2009, per capita GDP about $ 15,000."
To assume that US GDP is $15 trillion is more of an assumption that most will admit. So maybe the US GDP is $15 trillion but it may shrink in half if the financial sector gets into trouble. --a reader's comment to a ZH article.