Your theory hinges on “The Emperor is sooo powerful/evil that he…” which is lamer than shit.
He’s so powerful/evil that he can use force lightning. How is that lamer than shit?
Darth Vader is a top tier Force Wielder. To say that he can’t use Force Lightning because he’s somehow inept in comparison to Sidious is to claim Sidious is akin to a God which is lame. That’s like saying Sidious is sooo evil that he’s the only Sith who can have yellow eyes. That shit would be lame too.