I don't buy that Dark_Wolf. Vader wasn't more powerful than the Emporer. If he was he could have taken him out at any time. Palpatine was very focused on killing Luke at that point. He was allowing himself to be consumed with hatred and anger so that he could focus all his power towards Luke. Vader knew at that point that the only thing to do would be to grab the Emporer while he was distracted and send him down the shaft...all the while knowing that by handling the Emporer at that moment would kill him because of the intensity of dark force power Sidious was using. Which is what exactly happened.... Of course the Emporer never expected Vader to pull something like that, so he wasn't prepared for Vaders "aggressive" move of just brute strength.