Hi Longtimer,
Good post, and spot on. I think that if we are good historians we do not judge the past through our current cultural lenses (or at least try not to), but try to understand the cultural lenses of the past through which those people interpreted their world.
I agree that most empires have thought that those establishing the empire were superior to those they were adding to the empire, but, this has not always (often?) been based on conceptions of 'race'.
Also, some empire builders only thought they were superior in some aspects to those they conquered. A good example would be the Romans, who greatly admired the Greeks, and thought the Greeks were their superiors in many ways, (though not militarily nor in governance).
I would say that many imperialists did not base their sense of superiority on 'race'. The Romans are a good example. They did not consider themselves a separate 'race' from those they conquered in Italy. They had emperors from every part of the empire and many different ethnic origins. The Greeks considered themselves superior to other peoples, so Alexander's empire may have had some 'racial' biases (though perhaps not many), but the empires of Athens and Sparta did not, seeing as how the imperial subjects were other Greek polities. I don't think the Ottoman Empire was particularly preoccupied with 'race' for most of its existence. The Holy Roman Empire (mostly composed of Germanic and Italian peoples for most of its existence) does not seem to have had 'racial' issues.
You mention the Spanish Empire, as having notions of 'racial' superiority. I think it did in its later days, however, originally, differences of religion mattered much more to the Spanish than differences of 'race'.
Empires are complicated, and what the imperialists thought of their imperial subjects varied greatly across time and space, and from empire to empire.