Couple quick thoughts, as someone who bought a 2015 S5 with a seatpost that was broken down inside the frame. Basically I follow all of the guidelines in the Cervelo User Manual, which I cannot now find a good original copy of other than some links below:

• carbon fiber assembly paste: definitely; I apply it somewhat liberally to the seat post all around as well as to the face of the clamping wedge that sits against the seatpost.
• clamping wedge: the metal face of the clamping wedge was a little rough and nicked, so I pulled out the entire clamp from the seat frame and used a Dremel tool plus sand paper to clean up the face of the wedge and smooth all other faces out.
• Chamfer: the User Manual calls for a 45 degree chamfer to be cut to the trailing part of the seat post bottom - see here https://www.manualslib.com/...S5-Disc.html?page=33. I'm not a mechanical engineer but what I imagine in my mind that this does is reduce the force acting on one of the longest parts (the end) of the "lever" (if we imagine the seatpost is a lever with the clamping area as the pivot) that's otherwise unsupported internally (b/c the seatpost is hollow).
• The User Manual also specifies that the seat post clamp bolt threads should be greased - which I do. (Many/most Cervelo bolt threads are spec'd to be greased, IIRC?)
• While the manual calls for a max torque of 8Nm I now go slightly below that - both b/c the bolt threads are greased and also b/c of the paste. Last couple times I used 64 in lbs (or roughly 7.2 Nm) - roughly 10% below spec. So far so good, but I'm also only about 146lbs so probably not exerting max force compared to other riders. Maybe I could go lower, which I plan to test next time I move the seat by marking the position with a pencil and seeing if it moves over time. (10% is probably roughly within the error range of my torque wrench, so may not even be low enough to be real).

Here's also a good summary reference with some of this info https://www.bikesnbites.com/...-specs-and-settings/.

Good luck and hope this helps! Others might also have more info, especially through searching or posting in the main forum.