That's not at all how the effect of Lead exposure works. The research has found that Lead exposure causes a consistent loss of IQ points, with no evidence that the effect is limited to the low end of the IQ scale. Naturally, none of the analyses which determined this included the IQs of dead subjects.

https://www.ncbi.nlm.nih.gov/pubmed/8162884 https://ehp.niehs.nih.gov/doi/full/10.1289/ehp.7688