Generating series for the $E$-polynomials of $GL(n,{\mathbb C})$-character varieties

With G=GL(n,C), let $\mathcal{X}_{\Gamma}G$ be the G-character variety of a given finitely presented group $\Gamma$, and let $\mathcal{X}^{irr}_{\Gamma}G \subset \mathcal{X}_{\Gamma}G$ be the locus of irreducible representation conjugacy classes. We provide a concrete relation, in terms of plethystic functions, between the generating series for E- polynomials of $\mathcal{X}_{\Gamma}G$ and the one for $\mathcal{X}^{irr}_{\Gamma}G$, generalizing a formula of Mozgovoy-Reineke [MR]. The proof uses a natural stratification of $\mathcal{X}_{\Gamma}G$ coming from affine GIT, the combinatorics of partitions, and the formula of MacDonald-Cheah for symmetric products; we also adapt it to the so-called Cartan brane in the moduli space of Higgs bundles. Combining our methods with arithmetic ones yields explicit expressions for the E-polynomials of the irreducible stratum of GL(n,C)-character varieties of some groups $\Gamma$, including surface groups, free groups, and torus knot groups, for low values of $n$.