Sorry, there's a small terminological problem. The D3D9 rendering we have now is made by graphic card, so it is not software. Software one was in Doom/Quake.
But yes, there are a lot of actions made on CPU right now, which can be made by GPU. For example, vertex processing. And of course, the code does not utilize many of modern GPU features, it thinks we have Voodoo 3 overclocked by a thousand times or something like that.
As for Ogre, there are a lot of things to be done. As for effects - I even didn't try to implement them on Ogre's basis. But yes, modern "particles" way of effects implementation should allow to produce more complex effects with less fps influence. Support of more complex materials (including shaders, for example) will help even more.
Ogre will allow to import much more complex graphics entities (models, effects) than current EECH supports. But it means current EECH code would not be allowed to manipulate them directly, thus it should be modified. And all of these will take time - Ogre wrappers, EECH code modificication and new improved models, effects and other artist's creations.
But this is "infantry"'s thread! Please release!