Obi Wan would stomp Vader is far weaker then he was when he was Anakin(Pre Suit). As stated by George Lucas.
In writing the prequel trilogy, Star Wars creator George Lucas said he wanted the lightsaber combat to be "reminiscent of what had been done in the previous films but also something that was more energized. We'd seen old men, young boys, and characters who were half-droid, but we'd never seen a Jedi in his prime. I wanted to do that with a fight that was faster and more dynamic — and we were able to pull that off."