Better beginning preparation can bring up a whole national average to about the "gifted" level in the United States, in several countries that were formerly poor and backward,

http://nces.ed.gov/pubs2009/2009001.pdf

and several aspects of primary curricula in the United States are KNOWN to be suboptimal,

http://www.ams.org/notices/200502/fea-kenschaft.pdf

so there is surely still a lot of room for improvement in the United States.