GRE quantitative scores are not useful for high-IQ estimates because 6% of people get perfect scores.

A perfect GRE verbal score is roughly the 99.8th percentile, as can be inferred from the charts in this pdf: http://www.ets.org/Media/Tests/GRE/pdf/994994.pdf It shows that the percent of people with a perfect scores varies between less than 0.1% and 1.5%, depending on field, but it is usually 0.1% or 0.2%. (The 1.5% field was philosophy.) Because many non-native English speakers take the test, it's likely that one ought to adjust that percentile a bit lower.

That's among people applying to grad school, which is a higher-IQ group than the general population, but not by so much that 99.8th percentile among grad school applicants correlates to the 99.996th percentile among the general population, as that site (http://www.iqcomparisonsite.com/GREIQ.aspx) claims. That would be impossible assuming more than one in fifty people in the applies to grad school.

If we attribute a perfect GRE score to the 99.8th percentile, then looking up that percentile on the chart on the same page, we get an IQ score >142 for 1600 on the GRE.