Go to Yavin? Seriously? Not only will she have to fight through the hordes of Massassi, but she will have to face Exar Kun at the center of his power. I don't see Traya leaving Malachor. She isn't the type of persona that goes about killing whoever she wants. She's a manipulator. She'll send her servants to do it.
Yes of course but lets not forget that Xizor and Traya are not inseparably attached to their bases of operations. Traya can leave Malachor V (*gasp*) and come for Kun directly. As in, face him on Yavin 4. How does she do this you may ask? Well of course Kun is stronger than Traya. But is Kun stronger than Traya + Sion + the Three Sith Lords of Nadd's tomb? Plus some acolytes an assassins?
Now on the topic of Sion. Are we going to assume that he can come back to life after losing his head? That would be really really lame. And if he could come back, the best he could do is come back as a severed head. Now how useful is he going to be as a severed head?
Or what about the Massassi ripping him to shreds? I have never heard of Sion coming back from the loss of limbs (or his head).