II think that harder or stiffer masts do not directly relate to the static aspect - how does the sail look when rigged. For instance, the Loft sails can be rigged with either the stiffer SDM mast or the softer RDM mast. When rigged, those sails look quite the same. It's when they are used that you can feel a rather different behaviour. The loft sails recommend - especially for the larger sails - SDM masts for heavier riders and RDM masts for lighter riders.
The bend curve of the mast is more directly related to the static aspect, although it will also have it's influence on the dynamic aspect of the rig.
The bend curve can change seriously when using a longer extension.
In general, like already said by Farlo, longer extensions tend to give the mast more flex top.
While downhauling sails on Flex top masts, they release generally more the head (more loose leech) while changing less to the lower body (deep belly). That's the theory. The sail design cut needs to accompany this of course.
Wether a sail will feel softer or harder is again a combination of a lot of factors. In general, the more a sail can release the excessive air flows (more twist/flex), the softer it gets.
No too long time ago, there was an amazing thread on the Dutch windsurf forum about all this stuff, also referring to the recent speed contest in Luderitz.
For those of you who understand Dutch, I can highly recommend this to read.