I don't know how accurate of GDP per capita is that far in the past, as a certain number of assumptions must come into play. However, these numbers should give pause to anyone who makes broad statements about different populations' innate ability based on their present- or recent economic output, especially when such inferences are pushed thousands of years into the past.
The paper also has some estimates of population and population growth rates which should be interesting for population geneticists in calibrating their models.
Medieval England Twice as Well Off as Today’s Poorest Nations
New research led by economists at the University of Warwick reveals that medieval England was not only far more prosperous than previously believed, it also actually boasted an average income that would be more than double the average per capita income of the world's poorest nations today.