Yes, it is. If that king is ok with the Imperium's view of where he fits onto the Imperium's SOC scale, then you are correct. If that King considers Himself equal to the Emperor on the Imperium's SOC scale (and isn't crazy or an idiot), then you'd be wrong.No, it's not. Some independent system where the local ruler is a self-styled "king" is really just the same as some imperial baron on the SOC scale. The title changed but the status level remained the same. What you have to do is adjust the titles and such to fit the situation rather than change the SOC level.
Now, I understand that SOC is from the Imperium's point of view, and in that regard, I would be totally off base and wrong. On the other hand, I've been looking at SOC from outside the Imperium looking in, so maybe I've been tainted in that regard. I think that this is where Diplomacy comes into play.