from Erin Johnson:
The 2007 TIMSS does break out the US results by race. In 4th grade the US Asian subgroup performs only slightly less than the top Asian countries but by 8th grade the US Asian subgroup performs significantly lower.
4th grade US Asians: 582
4th grade Singapore: 593
8th grade US Asians: 549
8th grade Singapore: 599
Highlights from TIMSS (pdf file)
Note that the math scores of Singapore stay the same from 4th to 8th grade, but the US Asian subpopulation (as well as the entire student population) declines.
If math performance was purely an IQ phenom, we would expect that those 4th and 8th grade levels would be comparable, but this is not the case.
Also, there is evidence from Whitehurst that math curricula does matter. So it is not inconceivable that differences between math in Asia vs the US might account for performance differences.
Anecdotally, having used Singapore math it is easy for me to see why Asian math programs would be significantly better at enabling kids to learn math.