  Redshift space distortions caused by galaxy peculiar velocities provide a
window onto the growth rate of large scale structure and a method for testing
general relativity. We investigate through a comparison of N-body simulations
to various extensions of perturbation theory beyond the linear regime, the
robustness of cosmological parameter extraction, including the gravitational
growth index, \gamma. We find that the Kaiser formula and some perturbation
theory approaches bias the growth rate by 1-sigma or more relative to the
fiducial at scales as large as k > 0.07 h/Mpc. This bias propagates to
estimates of the gravitational growth index as well as \Omega_m and the
equation of state parameter and presents a significant challenge to modelling
redshift space distortions. We also determine an accurate fitting function for
a combination of line of sight damping and higher order angular dependence that
allows robust modelling of the redshift space power spectrum to substantially
higher k.
