I can't help but feel that the most important point (imho) is being missed by quite a few readers, although it has actually been made a number of times...
The SI cannot turn ANY companions, the SW can.
It doesn't matter whether it is Ashara or not; if a SW grunt can turn someone, we should be able to turn at least one of our companions by now, surely?
This cannot be right... it should be corrected... simples
I am prepared to accept "The Turning of Ashara", or whatever storyline you want to suggest, as a planned long term option. If that should be the case I would hope a choice in either direction, or even to stay "grey", would be possible; so that players have a chance to try the combination of their choice. Anyone considered being a LS SI and having Ashara turned DS?... Less likely, but you could possibly argue that you are manipulating your companion to do all the dirty work, while you keep your hands "clean". Don't get me wrong, I intend my char to be fully DS and I would want to turn Ashara if possible, and be able to kill her if I couldn't; but I accept and understand that many other players will have a different perspective for their character/companion interactions, and they should have a choice too.
I do understand the constraints that may make so many options for one npc difficult for Bioware to implement, however, so I think it unlikely more than DS or LS would become available, if at all.
Back to original point...
If SW can already turn a companion, then SI should definitely be able to do so... fix it please.