I've read this a billion times, but do not find it in my experience.
Switched from a Lemond Zurich steel bike to a Giant composite with easton 90 carbon seatpost and notice no difference in comfort.
I think it must be a placebo effect for some people?
If your seatpost breaks or needs replacing for some reason, then by all means go for a nice looking carbon post, but "upgrading" in search of comfort is not a worthy investment IMO.