never though much of this objection at all. Suppose they conclude that it would be best to wipe us out. If we accept the stipulation that their moral reasoning really is above and beyond what we are capable of comprehending then the situation is just: (1) It would be right for them to wipe us out; and (2) We wouldn't understand why.
That would be tough, but it's no great philosophical puzzle, and being nice to lobsters now won't help us if this ever comes to pass. :)
I got your point, but given its inductive nature, it doesn't hold: would you consider it OK for you to be the lobster?
I.e. you need higher standards to let the aliens wipe out humans than you need to cook lobsters.
Moreover, you assume a higher level of understanding for the aliens, hence admitting that you may not comprehend all the consequences of wiping out lobsters, that in the higher ethics, it may be wrong (= the laiens would never harm these lobsters). Hence, your decision to cook lobsters is not guaranteed to be right. It's even impossible to justify without a "maybe" inside.