Even if there were a race that was actually inferior, would it be okay to enslave them, or otherwise mistreat them? Or let's say instead that we had excellent genetic tests that predicted someone's intelligence. How should the identifiably stupid people be treated?

I think their preferences should still receive equal weighting. Peter Singer's fond of this quote by Sojourner Truth in relation to this point:

"They talk about this thing in the head; what do they call it?" ["Intellect," whispered some one loudly] "That's it, honey. What's that got to do with women's rights or Negroes' rights? If my cup won't hold but a pint, and yours holds a quart, wouldn't you be mean not to let me have my little half-measure full?"