I agree with Darth up til the recommendation using a 15-16 N-m wrench on a saddle clamp specced at 6.2 N-m. (I may be confused about this: the OP states that his saddle clamp is specced at 6.2 N-m, but is rated at 13.5 N-m). A couple of things about torque:
1) The idea is not necessarily to get the right torque, it's to get the right tension on the bolt. This stretches the bolt a bit so that it retains its clamping force in use.
2) Typically, the amount of stretch is a significant amount of the available elastic stress. That is, torquing to spec stretches 'em pretty near the point of plastic deformation
3) To get the proper bolt tension, the amount of torque needed varies depending upon thread and face condition, and lubricant (or locker) on threads
4) These variations mean that the torque spec is approximate. One significant digit. So 6.0 and 6.2 are practically the same.
5) For super-accurate tensioning, you measure the bolt shank length before and after tightening.
Point 2 is why I may be confused. If the torque spec of a bolt is 6.2 N-m, tightening to 13.5 N-m will typically deform the bolt. Also, there's a lot of bolts on bikes that are set up to torque to 6 N-m. This allows (for example) me to care one t-wrench in my toolbag to tighten handlebars and seatpost clamp both.
Carbon construction is optimized, and brackets and such don't have the same safety margin as the same bracket on a carbon frame. Soooo...
--> Use assembly paste
--> Torque to stated spec (6.0 N-m) will be fine.
If I couldn't get the saddle to stop squeaking with paste and torque to spec, I'd take it back to the dealer and ask what gives. More than doubling the torque might pull the bracket off the frame, destroy the frame, and kill the warranty.
Again, to the original question: a 6.0 N-m wrench will be fine (especially with assembly paste).
For the technically minded, a German engineer wrote about fasteners
here. Recommended.