A straight up fight has never been the Sith way. Darth Bane himself, creator of the Rule of Two, has stated several times that the tools of the Sith are cunning, betrayal, and deceit. It is the way of the Dark Side, always has been. Any Sith who believes otherwise isn't worthy of using the Dark Side. Again, this is shown by how Darth Bane handled Lord Kaan and the Brotherhood of Darkness. Also shown when Darth Bane fought against Lord Kas'im on Lehon at the Temple of the Rakata.
Point taken but being more cunning and deceitful doesn't mean you are more powerful.
Case in point Sidious didn't surpass his masters knowledge as he even tells Anakin that he never figured out how his master prolonged his life but that maybe they could figure it out together.