Ummmm.... last time I saw one, the North "Carbon Xtender" was made in exactly the same way and of exactly the same materials as the mast it extends.
So, it will bend by design.
Putting a rigid aluminum or carbon (with a different fiber axis bias and not designed to bend) mast base extension up inside a mast that is already reinforced for hoop strength
from the boom area to the bottom really makes the bottom of the mast very rigid and unable to bend.
I guess the std. extension lengths are:
Stubby .....0 amount of adjustment
Short..... 10-12 cm of adjustment
Medium.... 26 cm of adjustment
Long/Tall.... 46 cm of adjustment
So, if your sail specification calls for the mast to be 455-460 cm, use a stubby.
If your sail calls for 462-472 cm use a Short extension
If your sail calls out a 474-486 cm use a Medium extension
If your sail calls out 488 cm or greater (up to 506 cm) use a long/tall extension (unless your sail specifies a longer/stiffer 490 cm IMCS 28-30 mast.
Hope this helps,
P.S. As suggested above, this is for getting the optimum tune from your race/slalom sails on standard diameter 75-100% CARBON Race masts.
RDM masts/Low carbon economy masts used in freeride and smaller wave sails probably are not as sensitive to having the right mast/extension geometry.
The North Carbon Extender System works well in North Sails designed for that mast Xtension system, but does not work so well in sails from other lofts.