Lord Vader is definitely more powerful than Sidious why hasn't he overthrowed him? In the game of Episode III he does that in the alternative ending.
I don't think he is more powerful than Sidious, actually. I think once he became mostly machine, his power to use the force was diminished. For instance, he cannot use force lightning, like Sidious. If the measure of power within the force is measured by midichlorians, and Vader is half-machine, it stands to reason he has a lower balance of midichlorians than before. :::shrug::: Just my two cents ... !