  We investigate the impact of relativistic SZ corrections on Planck
measurements of massive galaxy clusters, finding that they have a significant
impact at the $\approx$5 -- 15% and up to $\approx 3\sigma$ level. We
investigate the possibility of constraining temperature directly from these SZ
measurements but find that only weak constraints are possible for the most
significant detections; for most clusters, an external temperature measurement
is required to correctly measure integrated Compton-$y$. We also investigate
the impact of profile shape assumptions and find that these have a small but
non-negligible impact on measured Compton-$y$, at the $\approx$5% level.
  Informed by the results of these investigations, we recalibrate the Planck SZ
observable-mass scaling relation, using the updated NPIPE data release and a
larger sample of X-ray mass estimates. Along with the expected change in the
high-mass end of the scaling relation, which does not impact Planck mass
estimation, we also find hints of a low-mass deviation but this requires better
understanding of the selection function in order to confirm.
