widespread availability of human genetic enhancement will lead to less variability rather than more

It's hard to say because of advantages to specialization. I think it's very unlikely that you would be able to just get all the gains without trading off something in other areas. This implies (to no surprise) that people genetically specialized for a particular class of tasks will be superior at them. And this implies that people, given the chance, will genetically specialize. To what degree is an interesting question.