I am Afrikaans. A language very similar to Dutch. My Sir is English and doen't understand Afrikaans. However, if he did and I had to address him, I have no doubt that I would use the formal 'U'. It not only clearly shows the dynamics of our relationship. It is a sign of respect. (Not simply because he is a dominant, but a sign of the respect I have for him as a person too.) And a very clear way for me to feel more submissive to him.