The ethnic makeup of Germany is 90%+ German. Look at the IQ bell curve for various races. Compare that with the racial makeup of the US and do the math yourself.
A more relative comparison is to compare all of the EU with the US and when you do that, it's a complete mismatch in terms of median income, middle class living standards, etc.
I'm not sure why people keep ignoring that, it is as if you could simply install the 'correct' economic policy in the central african republic and out would pop another germany.
Any country or economic system is only as good as the people in it. One of the primary reasons for american success is the high historical percentage of northern europeans, and lately some east asians. That's all about to change folks, say hello to the U.S. of Brazil.