I don't think that he is stronger than Vader. I think that he is in a game where he is the star and has to win by spamming force powers on steroids. I truly think that if Marek was not plot protected and Vader took him seriously in TUF I, Starkiller would not have made it to Sidious. In TFU II, they were fighting in a thunderstorm, and it took a ridiculous amount of Lightning to bring down Vader...So no, Starkiller is not above Vader.