  In the canonical tachyonic resonance preheating scenario, only an order one
fraction of energy density in the inflaton is transferred to radiation, due to
backreaction effects. One possible way to improve the energy transfer
efficiency is to allow for the perturbative decays of the resonantly produced
daughter particles, which serve as the "spillway" to drain the direct decay
products from inflaton and to reduce the backreaction. In this article, we
study two observational consequences of spillway preheating. The first is on
the inflationary observables: the scalar spectrum tilt $n_s$ and
tensor-to-scalar ratio $r$. The spillway scenario modifies the evolution of the
equation of state between the end of inflation and the thermal big bang. As a
result, it affects the time elapsed from inflation to the Cosmic Microwave
Background (CMB), as well as the fits of inflationary models and their
corresponding prediction for $n_s$ and $r$. We map out the equation of state by
systematically scanning the parameter space of the spillway scenario, and show
that the most efficient spillway scenario predicts a bluer spectrum, compared
to the tachyonic preheating scenario. Another consequence is the production of
high-frequency gravitational waves (GWs). Comparing the simulation results with
those of tachyonic preheating, we find that the existence of spillways leads to
sharper-peaked GW spectra with a mildly damped amplitude.
