For the record, these quotes from the DE and the DE endnotes suggest that he was continually growing in power, and I doubt such things were confined to his DE incarnation.
Even now, as Luke appears to fall under the Emperor's ever-expanding power, massive engines of destruction are inflicting death blows to the floating cities of the Calamari, long-standing allies of the Rebel Alliance...-- Dark Empire
But the Emperor cannot be overcome so easily. He gives Leia a painful demonstration of his ever-expanding power, crowing in triumph that he has finally captured the last of the Jedi!-- Dark Empire
Yes, he grows continually more powerful in the way of the Dark Side.-- Dark Empire Endnotes
Also, just for the lols, there's a source claiming that Sidious had boundless power:
Possessed of the boundless power of the Force and adept in the ways of the dark side, Emperor Palpatine was one of the most dangerous and evil Humans in galactic history.-- The Dark Side Sourcebook