  We use the spherical collapse model to demonstrate that the observable
average density of virialized clusters depends on the properties of dark energy
along with the properties of gravity on cluster scales and can therefore be
used as a probe of these properties. As an application of this approach we
derive the predicted virialized densities and radii of cluster mass structures
for a wide range of values of the cosmological constant (including negative
values) as a function of the turnaround redshift. For the value of
$\Omega_{\Lambda,0}=-0.7$ (with $\Omega_{m,0}=0.3$) preferred by $\Lambda$
sign-switching models ($\Lambda_s\text{CDM}$) proposed for the resolution of
the Hubble and $S_8$ tensions, we find an amplification of the density of
virialized clusters which can be as large as $80\%$ compared to \plcdm for a
turnaround redshift $z_{\text{max}} \gtrsim 2$. Such an amplification may lead
to more efficient early galaxy formation in this class of models in accordance
with the recent findings of JWST.
