It would be quite interesting to see the math on it, and sivir does make good use of all the stats. I think the main reason I'm not too fond of building triforce on champions is that I'm not that great at managing the procs. Like I think I could be doing a lot more damage on corki when I have a triforce on him, but I kinda mash all the buttons and ya. Though he can get all the procs he needs from his rockets pretty much. It's probably more that I feel like I can just do a lot more damage with an IE than with a triforce. I am a graves player, so I am perhaps used to hitting quickdraw and watching those crits pop up.
I'll have to give it a try on her, since her W has such a low cd that you could make good use of it.