Does the seatpost matter?

[TxDoc]

They do actually make a difference.

+1.
But, remember that in theory - to be able to absorb vibrations effectively, the carbon shaft must be free, not inside the seat tube. So it does really make a difference whether your setup allows only a handful of seatpost to show, or several inches of seatpost are visible atop of the seat tube (greater vibration dampening effect)...

[warneral]

I rode on my carbon seatpost today. Too lazy to really get a good idea if it mattered b/c I left my favorite saddle on there. I definitely notice an improvement in vibrations but I would really need to test apples to apples with the favorite saddle and both seat posts!