I had in interesting though today as I worked on a one-shot. It's entirly possible that I'm reading too much into things, but I think it's an interesting point, if not necessarily important.
I think that Sui-Feng's instance on using honorifics when speaking to and about Yoruichi says a lot about the nature of their relationship.
From Sui-Feng's perspective, it potentially speaks volumes about the way she perceives Yoruichi, but mostly herself. Yoruichi doesn't mind that people don't refer to her with respectful titles, and in fact insists that people tone down formalities (but not do away with them entirely). Sui-Feng finds herself in an interesting logical paradox; does she respect the wishes of the person that she was told to serve, or adhere to the protocol that she was been taught? She's been told that Yoruichi is basically a deity, someone to be revered, and yet that same person is telling her not to be so formal. In choosing to to continue to use titles, it says to me that Sui-Feng sees Yoruichi as more of a concept than a person. If she saw her as a person, she would have respected Yoruichi's wishes.
There is also the more innocuous possibility that Sui-Feng sees it as a pet name of sorts. She's the only person that calls Yoruichi that, so she may see it as a sign of their relationship.
Now, as for Yoruichi's side, her abiding by the title says to me that she either likes it or doesn't care. Yoruichi has show herself to be a very pragmatic person, and she could probably tell that breaking Sui-Feng of her habit was going to be more trouble than the end result was worth. On the other hand, its possible that she actually likes it and that is why she didn't press the matter further.
What do you guys think? Am I on to something or am I simply seeing mirages in the YoruSui dessert?
(I realized upon writing all this that there isn't a single canon instance of Sui-Feng calling Yoruichi "sama" outside of the flashback and a single time when she broke down, which can argued to be the result of emotional instability. This essentially shot everything I wrote all to hell. In that case, I think the fandom's instance on continuing to have Sui-Feng use the title is an interesting point into itself.)