  Cosmic-ray acceleration at non-relativistic shocks relies on scattering by
turbulence that the cosmic rays drive upstream of the shock. We explore the
rate of energy transfer from cosmic rays to non-resonant Bell modes and the
spectral softening it implies. Accounting for the finite time available for
turbulence driving at supernova-remnant shocks yields a smaller spectral impact
than found earlier with steady-state considerations. Generally, for diffusion
scaling with the Bohm rate by a factor $\eta$, the change in spectral index is
at most $\eta$ divided by the Alfv\'enic Mach number of the thermal sub-shock.
For $M_\mathrm{A}\lesssim 50$ it is well below this limit. Only for very fast
shocks and very efficient cosmic-ray acceleration the change in spectral index
may reach $0.1$. For standard SNR parameters it is negligible. Independent
confirmation is derived by considering the synchrotron energy losses of
electrons: if intense nonthermal multi-keV emission is produced, the energy
loss, and hence the spectral steepening, is very small for hadronic cosmic rays
that produce TeV-band gamma-ray emission.
