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I don't know the answer to this question, but you may be able to determine it yourself by looking at the source code for plethysm. You can do this with .plethysm??, as in

sage: Sym = SymmetricFunctions(QQ)
sage: s = Sym.schur()
sage: s2 = s[2]
sage: s2.plethysm??

Also (I apologize if you've already checked this) the Wikipedia page on plethysm is very short, but includes a cryptic comment which may help you decide if your guess is correct or not. Good luck!