Calculation: Kids differ by 5 to 15 IQ points on average. Assuming a symmetric distribution, that means that the smartest kid would be 2.5 to 7.5 IQ points smarter than the average kid, while the dumbest would be the same amount dumber than the average.

Actually, I'm not sure whether "5 to 15" isn't meant to describe the standard deviation, in which case screening lots of embryos might even produce much higher gains.