Sanji was unable to losen Kings grip and he couldn't stop king from ramming him into the ground either. Sure he is way more durable with his RS, but he also gets a speed boost and speedier kicks should result in stronger kicks as well.
sanji wasn't trying to loosen kings grip as he was focusing on momo nothing indicates that he was and when King imbedded his beak after holding his neck he used his momentum of his flight to cling sanji on his beak which i doubt zoro or luffy would have been able to restrict in the same situation. so by no means was he overpowered.
Overpowered is kinda incorrect since he had to grab him by his neck to really able to imbed his beak.. so that's not over powering and RS doesn't provide strength.. it provides durability..
King's vivercard hyped his beak as he uses it to Deflect attacks and pierce his opponent which he couldn't against RS
Even so, Sanji could have dodged Kings grab couldn't he? Either his CoO is not enough against King, or King was just too fast to react to avoid being grabbed. Even if he looked away that shouldn't nerf Sanjis CoO at all. Take a look at his reaction to Katakuris jelly bean, he still dodged him without knowing that he would attack him. And now while King used his beak in his zoan form against Sanji, Sanji clearly tried to get away from him, but he couldn't. Thats what overpowering someone is for me. Here is the Panel to show you what I mean.