We study multiplicities $a^{dλ}_{μ,(dk)}$ of highest weight representations $\mathbb S_{dλ}(\mathbb C^n)$, $λ\vdash pk$, of length at most $p$, in $\mathbb{S}_μ(S^{dk}(\mathbb C^n))$, $μ\vdash p$, so called plethysm coefficients, as $d$ tends to $\infty$. These are given by quasi-polynomials, which in the case of $S^p(S^{dk}(\mathbb C^n))$ can explicitly be computed by Pieri's rule. We show that for all but a finite, explicit list of $λ$'s the leading term is in fact constant and that $$ a^{dλ}_{μ,(dk)}\sim \frac{\dim V_μ}{p!}c^{dλ}_{p,dk} $$ as $d\to\infty$. In particular, we answer a conjecture of Kahle and Michałek, going back to Howe.