UKH

A few weeks ago I bought a second hand bike with skinny steel frame, a carbon fibre fork and drop bars as a replacement for a cheap hybrid. After fitting a new stem, wider tyres and a saddle which actually fits me, it feels much more comfortable than the hybrid during rides on my well rutted local roads (some bordering on tracks), but I'm wondering if it's worth replacing the aluminium seat post with a carbon fibre one to further improve comfort.

According to some enthusiasts on bike forums carbon posts help to reduce road "noise", but I'm wondering if this is just a justification for spending loads of cash on bike bling. So, do carbon posts really improve comfort? And, if so, are there any decent ones around that don't cost an arm and a leg? I know the most expensive ones tend to be very light, but weight, within reason, isn't important to me. 

Wouldn't have thought you "needed" a carbon seat post on a steel bike - can make a difference on a stiffer aluminium bike.

I've got both carbon and alloy seat posts and can't say I notice much difference between them, tbh.

You used to be able to get carbon-wrapped alloy posts, which were sort of a "best of both" price/performance solution.

If you do get a carbon post, get some carbon grease and a torque wrench - over-tightening the clamp gets expensive!

Yes, carbon fibre parts absorb considerably more road "buzz" than metal ones, but I know of more snapped carbon posts than alloy ones. So personally I chose to use an alloy post because I don't fancy a carbon fibre enema.

I've never seen a snapped carbon post. Seen a couple of bent alloy posts. 

Yes, a carbon post would be a fitting compliment to your carbon forks as they do absorb a lot of shock. I used carbon posts for over 20 years on a hardtail mountain bike and could often feel them flexing to absorb big bumps as well as small. 

Here is your source for a wide range of quality, affordable carbon parts: https://www.carboncycles.cc/

You can get carbon posts that are split in the middle and act as a leaf spring, also skinny titanium posts with a shim are good at softening out the ride however going tubeless and having lower pressure in a wider tyre is generally cheaper and has more added benefits, one being a smoother ride.

By extension, just fitting latex inner tubes will give you a smoother ride and pairing them with a nice cotton-wall tyre like a Veloflex Open Corsa or the Vittoria equivalent (same factory) will give a delightful ride and a lovely ringing sound but they're not the most robust tyres if you commute.

Snapped my one (and yes I did use a torque spanner)

Well yes you could do something totally different and get different / worse results but I'd stick with my original plan.