In addition to this, the fact that G0-T0 could use the HKs as a tool for intimidation makes betraying him sound like a more beneficial prospect. Not many people like to be ruled by an Iron Fist, especially criminals. If G0-T0 starts going around saying "Work for me or die!" people won't respond well. Like you and Garfield have noted in the past, criminals and underworld types want to be free. If G0-T0 starts putting pressure on them, guys like Visquis will be easier to recruit for Traya. All she has to do is say that the Sith will leave the Exchange be if Visquis/other Exchange cohort helps her take down G0-T0. And Traya is quite the manipulative witch, convincing a few criminals shouldn't be too difficult for someone like her.
But unless someone has an example of when these Sith Assassins have actually done anything, they'll remain just a myth. G0-T0 can easily claim that any death caused by the Sith Assassins was actually just him eliminating someone who joined Traya's side. Traya isn't exactly a propanganda machine. G0-T0 knows how to rule a crime syndicate.
And G0-T0 doesn't have to say "Work for me or die." He can stage a betrayal, have his HKs kill the traitor, and plaster it all over Nar Shaddaa. In other words, don't betray the Exchange. And these people in power are unlikely to give up their positions in the first place. They have power and want to keep it.
And I don't think Traya meeting with anyone in person is such a great idea.