  The bulk motion of galaxies induced by the growth of cosmic structure offers
a rare opportunity to test the validity of general relativity across
cosmological scales. However, modified gravity can be degenerate in its effect
with the unknown values of cosmological parameters. More seriously, even the
`observed' value of the RSD (redshift-space distortions) used to measure the
fluctuation growth rate depends on the assumed cosmological parameters (the
Alcock-Paczynski effect). We give a full analysis of these issues, showing how
to combine RSD with BAO (baryon acoustic oscillations) and CMB (Cosmic
Microwave Background) data, in order to obtain joint constraints on deviations
from general relativity and on the equation of state of dark energy whilst
allowing for factors such as non-zero curvature. In particular we note that the
evolution of Omega_m(z), along with the Alcock-Paczynski effect, produces a
degeneracy between the equation of state w and the modified growth parameter
gamma. Typically, the total marginalized error on either of these parameters
will be larger by a factor ~ 2 compared to the conditional error where one or
other is held fixed. We argue that future missions should be judged by their
Figure of Merit as defined in the w_p - gamma plane, and note that the
inclusion of spatial curvature can degrade this value by an order of magnitude.
