Why would you disregard the novel? That provides far more in-depth coverage, and besides, this is DE Sidious, who canonically grew in power.
Marek also never came close to beating Sidious. Sources confirm he could have never beaten Palpatine, and it's evident the Emperor held back in that false fight earlier on to goad him into joining the dark side (IIRC, even Galen admits this in the game). Regardless, the fact that Galen ended up dying from Sidious's Lightning even when Sidious himself was utterly unharmed pretty clearly shows that Sidious >>>> Marek.
Yes, there is. From The Ultimate Visual Guide: Updated and Expanded:
"Resurrected in a youthful clone body, Palpatine does not reveal himself immediately. Studying the dark side of the Force to become more powerful, his education results in three manifestos: The Book of Anger, The Weakness of Inferiors, and The Creation of Monsters."
From the Dark Empire Sourcebook:
"Soon he was ready to strike. Fully healed and in greater control of the Dark Side than ever, he finally acted to end the Mutiny."
Palpatine himself also states:
"You've grown very strong in the Force since we last met… But then, so have I!"
If this is Viti 3.0 then this is a fuster cluck and team 1 curbstomps em and then pennitrates so hard their innards decintegrate and they implode. Revan contended with Vitiate at a point where he was still growing in power, and was noticebly holding back, where this would be a far stronger Vitiate and wouldn't stand a chance.