It seems some recent (kernel?) updates have mitigated the issue a lot when
running "redshift -t 4500:4500 -l 1:1", the transition phase is now free of
stutter. After that, there is still regular stutter though for subsequent gamma
adjustments by RedShift when Compton with vsync is active.

What is really interesting: There is kwin-lowlatency as a kwin fork with vsync
adjustments:
https://github.com/tildearrow/kwin-lowlatency
With its compositing, the performance issues of atomic modesetting seem to be
entirely missing. There is zero stutter when opening windows etc., and also
continuous RedShift adjustments don't stutter. At the same time, there is zero
tearing.