It could have a dramatic affect comfort, I think, but it depends on how much seatpost extension is available. You'll get max impact at +5" of extension; 3" or less and I think it wouldn't be as noticeable.

this is just based on my experience, and I've never seen any study or even mention of extension as an element of seatpost comfort, but clearly it must matter. There's a Velonews seatpost test talking about bump deflection and vibration damping out there you can check out, but again, no mention of the extension lengths they tested, or if they were even uniform.

the nice thing is that you can get a carbon post for relatively little and give it a shot.