I'd like to know if you regard the phrasal verbs 'come off' and 'come over' as synonymous in the sentence below or whether there is any difference in meaning. Thank you.

She comes over as an efficient businesswoman.
They're probably synonymous, but I have a slightly negative feeling about "come off", as if it isn't really true, or as if it is used with an adjective that carries a negative meaning.

[The man selling magazines came off as a legitimate salesman, but he was a scam.]

[The newly-crowned princess came over as a breath of fresh air.]

Let's see what other natives have to say about it.

it seems to me that you have a point there. I found an example in my dictionary where the verb 'come off' has a slightly negative connotation:

He comes off as quiet, but he's got a great personality.

The phrasal verbs 'come over as' and 'come across as' are synonymous. But I'm doubtful about whether 'come off' would have the same meaning as 'come over as'.
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I'm glad that my instinct didn't cause you any confusion.
I don't think I've ever heard anyone say "come over" in this context. "Come across" would be more common.
over doesn't seem right to me either.

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This is a dictionary definition (Longman Dictionary of Contemporary English) for 'COME OVER' :

4 if someone comes over in a particular way, they seem to have particular qualities [= come across]:

--- He didn't come over very well (=seem to have good qualities) in the interview.

come over as

--- She comes over as a very efficient businesswoman.
Well, as I said, I don't think I have ever heard it used that way. Maybe it's British?