I'm not sure why we would conclude Ben Kenobi's Saber mastery increased when he had no one to spar with for almost 20+ years. I imagine his understanding of the Force increased marginally, but he has zero showings to support being at or above Dooku's level. From a purely technical standpoint, Dooku is the best pure duelist in the PT, and often loses only to true Force titans. Obi isn't one of them.