Alright, that's it. I'm going to out this to bed once and for all. Vader, in no way, shape, or from, scales to Sidious.
1. The Galen Marek argument. Vader fought Galen who then fought Sidious. This doesn't mean anything. In the xbox version, Galen outright says Sidious was holding back in their fight. In the novel, Galen chucks a box at him and Palps acts all defeated. In the wii version, the only version that depicts Sidious as even remotely trying in their fight, Sidious emerges victorious without a scratch on him after all is said and done, while Vader is left limping. Years later, Galen would electrocute the hell out of Vader while bloodlusted and only incapacitate him. Palp zaps him for five seconds and he dies.
2. The Luke Skywalker argument. The novelization explicitly states that both Vader abd Luke were going all out. Immediately afterward, Palp leaves Luke writhing in agony on the floor and Vader dies whilst saving him. The force lightning wasn't even directed at him for crying out loud.
3. Statements. "Vader is 80% of Sidious, Dark Empire Luke is weaker than Vader, etc." One, these statements are inconsistent. Not only is Vader depicted as fodder nrxt to his master, people who beat Vader are depicted as fodder in comparison. See the above points. Not to mention, Palpatine is an overconfident bastard who loves to belittle people, so his statement to Luke can easily be taken as that. This statement would also create circular scaling, as Dark Empire Luke is comparable to Dark Empire Palpatine who is stronger than ROTJ Palpatine who killed Vader, despite Dark Empire Luke being weaker than Vader. See the problem?
Vader is staying where he is.