  Quantum metrology protocols exploiting ensembles of $N$ two-level systems and
Ramsey-style measurements are ubiquitous. However, in many cases excess readout
noise severely degrades the measurement sensitivity; in particular in sensors
based on ensembles of solid-state defect spins. We present a dissipative "spin
amplification" protocol that allows one to dramatically improve the sensitivity
of such schemes, even in the presence of realistic intrinsic dissipation and
noise. Our method is based on exploiting collective (i.e., superradiant) spin
decay, an effect that is usually seen as a nuisance because it limits
spin-squeezing protocols. We show that our approach can allow a system with a
highly imperfect spin readout to approach SQL-like scaling in $N$ within a
factor of two, without needing to change the actual readout mechanism. Our
ideas are compatible with several state-of-the-art experimental platforms where
an ensemble of solid-state spins (NV centers, SiV centers) is coupled to a
common microwave or mechanical mode.
