  The observed rest-UV luminosity function at cosmic dawn ($z \sim 8-14$)
measured by JWST revealed an excess of UV-luminous galaxies relative to many
pre-launch theoretical predictions. A high star-formation efficiency (SFE) and
a top-heavy initial mass function (IMF) are among the mechanisms proposed for
explaining this excess. Although a top-heavy IMF has been proposed for its
ability to increase the light-to-mass ratio (\(\Psi_{\mathrm{UV}}\)), the
resulting enhanced radiative pressure from young stars could decrease the star
formation efficiency (SFE), potentially driving galaxy luminosities back down.
In this Letter, we use idealized radiation hydrodynamic simulations of star
cluster formation to explore the effects of a top-heavy IMF on the SFE of
clouds typical of the high pressure conditions found at these redshifts. We
find that the SFE in star clusters with solar neighbourhood-like dust abundance
decreases with increasingly top-heavy IMF's -- by $\sim 20 \%$ for an increase
of factor 4 in $\Psi_{\mathrm{UV}}$, and by $50 \%$ for a factor $ \sim 10$ in
$\Psi_{\mathrm{UV}}$. However, we find that an expected decrease in the
dust-to-gas ratio ($\sim 0.01 \times \mathrm{Solar}$) at these redshifts can
completely compensate for the enhanced light output. This leads to a
(cloud-scale; $\sim 10 \, \mathrm{pc}$) SFE that is $\gtrsim 70\%$ even for a
factor 10 increase in $\Psi_{\mathrm{UV}}$, implying that highly efficient star
formation is unavoidable for high surface density and low metallicity
conditions. Our results suggest that a top-heavy IMF, if present, likely
coexists with efficient star formation in these galaxies.
