Valkorion's supremacy, a comparison.
There is a most telling difference between Valkorion and Darth Sidious, it is seen when they engage powerhouses in combat, for example when Darth Sidious faces Master Yoda, it is indicated in a plethora of sources that the fight is mostly a stalemate:
Yoda's knowledge of the Force makes him just as powerful as Darth Sidious.
- Star Wars: Mysteries of the Jedi
There came a turning point in the clash of the light against the dark.It did not come from a flash of lightning or slash of energy blade, though there were these in plenty; it did not come from a flying kick or a surgically precise punch, though these were traded, too.
- Revenge of the Sith Novelization
YODA unleashes a ferocious assault on PALPATINE, causing him to almost go over the edge. The Dark Lord drops his lightsaber but recovers with a BLAST OF ENERGY from his hands that surrounds YODA. YODA is deflecting the Sith Lord's lightning bolts.The energy bolts begin to arc back on the Emperor. It looks as if the Dark Lord is doomed.
- Revenge of the Sith Script
Yoda caught it. The blue energy built into a glowing ball in his hand, ready to throw back at the Sith Lord the moment his attack stopped. But Palpatine didn’t stop; the Force lightning came in a steady crackle, building more and more, until neither of them could hold it any longer, and the blast knocked them both out of the pod.
- Revenge of the Sith Junior Novel
These titans continue clashing until Yoda is downed, but hardly defeated.
- Star Wars Trading Card Game
Sidious battles with passion and fury, only to be matched every blow by Yoda's calm, measured skill.
- Star Wars: Beware of the Sith
Master Yoda is obviously the most powerful Jedi of all time, until the likes of Master Luke Skywalker. But then we have Revan, who is noted to be one of the most powerful Force weilders in the entire mythos:
"Revan was one of the strongest Force-weilders ever."
- Chris Avellone
Infact, Revan is noted to be more powerful than any Jedi, including the likes of Master Thon, Odan-Urr, and all others:
Revan has become more powerful than any Jedi has ever dreamed of.
- Star Wars: The Old Republic
Revan is again stated to be one of the most powerful Force users ever:
"Revan travels to a very iconic Yavin IV to go to battle and release the actual Emperor, one we thought who was defeated not long ago. The player is pulled into this struggle of two of the most powerful Force users, like ever, in the history of the Old Republic, if not beyond - Revan and the Emperor."
- Michael Backus
He has been capable of ashing Sith Lords such as Darth Nyriss, who has Force lightning feats even Count Dooku hasn't replicated. This was over three hundred years before his prime. He also faced two strike teams of some of the most powerful Jedi and Sith in the galaxy, back to back, and was barely defeated on both occasions.
Yet even he is not capable of standing against Emperor Vitiate:
"...The Emperor will prove far too powerful for Revan, or anyone else."
- Darth Marr, Star Wars: The Old Republic: Shadow of Revan
Noting that this is merely Vitiate's resurrected spirit and not the much more powerful Valkorion.
So whilst one can certainly state that Yoda is more powerful than Revan, thus making Darth Sidious' stalemate less of stand out. It can't overcome the fact that Valkorion was beyond being far too powerful for Revan, the gap is even more significant.