Starkiller aka Galen Marek at full potential
From what I watched on youtube (the authors and designers of TFU), Starkiller was meant to "be a wrecking ball of the Force" - no Force user had used the Force like Starkiller. Starkiller was meant for destruction so in fact he was not a true Sith. Vader and Sidious had misjuged the Force abilities Starkiller aka Galen Marek could summon. Starkiller had the uncanny ability (due to his training by Vader) to litterally "unleash the Force" for whatever the reason. Think of it as this way, only full potential Luke has the ability to stop two Sith Lords. Look what Luke did to Caedus in LOTF Inferno. Starkiller did that to Vader and Sidious in TFU due to his insane training.
Nobody can be certain how powerful Starkiller aka Galen Marek would be at full potential. My guess he would have A LOT more strength (in the Force) than Darth Caedus - possibly close to Luke, but that's my own opinion. Starkiller only knew Shien lightsaber form, but what if he was taught Makashi or Soresu for that matter? What are your opinions?