Thanks Ola for your thoughts concerning my question. I guess I've been lucky extending my older SDMs (Fiberspars), but I think that has to do with the fact that they tend to be a little stiffer in the bottom to mid section.
It is good to know that the 100% Gulftech 400 RDM extends well. I anticipate buying a new Hansen Sails 400 RDM very soon, and its my understanding that Bill Hansen will be patterning his mast line very similarly to the Gulftechs (same overall flex characteristics). With my 5.0 sail I need about 20cm of extension. Not too much lift, but more than required for my other RDMs.