Which of the following is a proper paraphrase of "I expected better of Sarah"?
a. I thought Sarah would behave better.
b. I thought Sarah would have behaved better.
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I'd say both of OP's alternatives are perfectly acceptable for the cited context, but possibly some pedants might want to argue with the logic of that position (because both the speaker's thought and Sarah's behaviour were in the past - superficially a better match to OP's second version).
In practice, though, few native speakers would think OP's first version was at all unusual. But consider this very similar example...
1: I thought1 you would like him
2: I thought you would have liked him
...where #1 could be used with either of the two possible implications ...but actually I was mistaken OR ...and I was proved right. But the Perfect form #2 can only normally be used with first of those two implications (I thought it would have happened, but it didn't).
It would take a very contrived context for OP's first version to carry the implication that the speaker's expectations had been confirmed (i.e. - that Sarah's behaviour did improve). In theory the words could have that meaning (as with my alternative), but that would be a very unusual way of expressing it.
1 Note that I highlighted would like / have liked in my examples to draw attention to the different verb forms. But in actual speech, heavy stress would be placed on either would or like/liked to convey the first possible meaning (speaker acknowledging that he was mistaken). OR heavy stress would be placed on thought to convey the second sense (speaker triumphantly announcing that his expectations have proved correct).