|Pete, Matthew, Cass,|
There is first a mistake in the question. The difference between a dolphin's consciousness (which does not have any "reasoning" ability at all) and a humans is not a difference of degree, but a difference of kind. Reason is not possible without volition, and volition and instinct are incompatible. It is not possible for a pre-programmed pattern of behavior (instinct) and volition (all behavior must be chosen) to both exist in the same consciousness. It is one or the other. The dolphin's is instinct; very complex instinct, capable of learning (read being programmed) to a very high degree, but rationality is impossible to the dolphin.
While there are obvious differences in the mental abilities of human beings, the ability to reason either is or isn't. Some people are able to perform some kinds of mental functions faster or more completely than others, or more easily, but even a fairly dumb person can learn the rules of logic and apply them to whatever they reason about. We often make the mistake of equating certain mental abilities with better reason, like the ability to perform certain mathematical operations, or having a good memory, or imagination, or a particular talent for writing, or music, for example. These are not abilities to reason.
A superior race might have hugely advanced stores of knowledge and may have developed unimaginable techniques for understanding and dealing with the physical world, but as for the ability to reason, they will either be able to do it, or they won't.
As for the moral question, if they can reason, it means they are volitional creatures. Another mistake we all commonly make is the tacit assumption morality pertains primarily to how beings treat each other; but the fact is, morality pertains primarily to our own behavior. It is immoral for a volitional being to seek the unearned and undeserved; his very nature is against it and gaining what his own effort and choices do not entitle him to is against his own nature, and that nature will make him feel guilt for attempting it. It is unlikely a race that has not discovered these moral principles would survive long enough to develop the technology to reach this planet--they would long before that use that technology to destroy themselves; a principle many on this planet are working very hard to demonstrate is true.
Obviously, there is no moral "right" to enslave or exterminate other moral beings, that is, other volitional beings. Neither superiority or inferiority change the nature of rights. The one question I have is why would any race, observing human behavior, conclude we are rational beings?
One caveat: the ability to reason, no matter how "great," is no guarantee it will be used correctly or morally. See human history.