Katsuyu would be loyal to no one. No reason to assume that she'd have to pick between Sakura and Tsunade when she can just not fight. Not to mention they can each summon 5% of the total slug from the forest, so they each have their own slug.She hasn't surpassed Tsunade yet. Their moveset is the same, but Tsunade has better taijutsu and more experience. In a battle Katsuyu would stay loyal to her, therefore she still beats her student.
Experience doesn't mean anything at all.
The only thing you have a point with is the better Taijutsu, but Sakura hits harder than Tsunade does, and probably has more chakra in her seal than Tsunade does.