  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 $n$.
