Why then do Africans underperform even in western nations where they aren't malnourished and are generally...?

Black culture encourages ignorance. For example, rap music, ing, thug life, Ebonics, Flava Flave, walking around with your pants falling down, and so on. If a black male dresses well and is articulate, holds 2 jobs, goes to college, and speaks perfect english, he is called an uncle Tom or a sell out. As long as this is the prevailing trend, the young or impressionable blacks will not succeed.