suggests we should be mixing races to get the best genetics from every group.

Not quite. You are thinking of breeding people to develop a trait (in this case, intelligence) and are correct that you want diversity in your breeding stock. However what that diversity gets you is not just top-end results. It gets you variance -- basically, you'll get a few geniuses and a lot of idiots.

In animal breeding that's not a issue -- you kill off (or prevent from breeding) all the failures and just keep the very few top results. For humans that would be... problematic.

So if you encourage greater variance in outcomes and you keep all of them, the question becomes who breeds faster: idiots or geniuses. Let me point out that I'm not optimistic about that question.

By the way, empirically people with both black and white ancestry have average IQs between the pure blacks and the pure whites. This seems to indicate that you don't get much by cross-breeding.