Opener
I'm not the type to mess around here, so I'll get right to this. Darth Plagueis, is simply not as powerful as Exar Kun; Hier to the Golden Age of the Sith Empire and the greatest of the ancient Dark Lords.
Exar Kun's spirit is repeatedly portrayed as the heavyweight in the power struggle between himself and his new Sith Apprentice, the self-proclaimed Dark Lord of the Sith Kyp Durron.
At first, Kyp Durron fails to overwhelm Corran Horn:
The Force blow he aimed at me glanced off my chest, yet it was strong enough to bounce me off the corrider wall. I managed to catch myself against the rough stone, but not before I'd slid half-way down the wall.
"You are not my master!" Kyp pointed back toward the dining room, "He is not my master! What good is it being a Jedi if we do not act?"
"What good is it being Jedi if we don't act responsibly?" I hauled myself upright, "Remember Kyp, the 'no good Jedi' kicked Kun's butt.." Kyp struck at me again through the Force but I expected it this time, I absorbed enough of it to let me create a shield.
- Star Wars: I, Jedi Audiobook
Once joined by the spirit of Exar Kun, however, Corran Horn is slammed into a wall and knocked clean out. Exar Kun also simultaneously clouded this clash from the senses of Master Luke Skywalker and every Jedi in the hall merely fifteen meters away:
"You're involved with someone who lost a long time ago, don't compound his error."
"And who will stop me?" I hesitated because Kyp's words seemed to echo within themselves, we were not alone, which meant Kyp's mentor had come to aid his apprentice.
"I will, if you make it necessary." An ancient sneer of contempt twisted Kyp's features.
"Puny Jedi, you are of no concern to me." Even though I had braced myself for another attack, it did no good. Kyp's previous blows were like light breezes compared to this full-on gale. I slammed back into the wall with teeth rattling-impact.****
"What happened to you?"
"Kyp didn't like the menu." I winced as a twing rang through my ribs. "We had a discussion in the hallway, you didn't feel anything?" Heads all around the hallway shook, and I felt a cold dread begin to congeal in my stomach. If Exar Kun could mask the attack on me in such a way that Master Skywalker could not feel it barely fifteen meters away, then we were up against something more powerful than I'd cared to imagine existing.
- Star Wars: I, Jedi Audiobook
Luke Skywalker regards Exar Kun as a greater threat than Kyp Durron:
His best students were going sour, getting impatient, trying to push the limits of their abilities. But he had sensed a greater, deeper menace that vibrated within the very stones of the Great Temple itself. . . evil, and well hidden.
- Jedi Academy Trilogy Vol.2: Dark Apprentice
Spoiler:
NOTE: That is two confirmations from the two primary sources on the matter that Exar Kun's spirit can cloud Luke's senses from discovering him regardless of them being pretty much right on-top of each other the entire time, with Luke knowing full well somethimg evil was undermining his praxeum.
This is all proven by the fact that the spirit of Exar Kun had possessed and corrupted him, and could actively exert control of his mind, even across the galaxy:
And then the haunting voice of Exar Kun rose to a wail in his mind, an utterly forlorn scream as if he were being torn out of this universe and exiled to another place entirely, where he could torment Kyp Durron no more. Kyp snapped backward in his control seat as if an invisible tow cable had been severed. His arms and head dangled like a puppet with suddenly snipped strings. The cool wind of freedom whistled through his mind and body. He blinked his eyes and shuddered with revulsion at what he had been about to do.
- Jedi Academy Vol.3: Champions of the Force
Exar Kun in spiritual form had also torn Luke Skywalker's spirit from his body, and was binding his spirit with his own power, something he maintained constantly at the same time as controlling Kyp Durron:
With a wheezing cough as he expelled long-trapped air from his lungs and drew in a fresh breath, Master Skywalker groaned and sat up on the stone platform."You've-done it!" Luke said, gaining strength from each lungful of cool, clean air. The new Jedi Knights surged toward him. "You have broken the bonds."
- Jedi Academy Vol.3: Champions of the Force
Later she became one of Skywalker's Jedi candidates and joined his other students in defeating the spirit of Dark Lord Exar Kun, protecting Luke's body and freeing his spirit.
- Ti, Kirana biography, The Complete Star Wars Encyclopedia
At this point, we need to recognise just how powerful both of them are. Kyp Durron having turned to the dark side meant he could become even worse than Darth Vader, per Luke:
If Kyp had fallen to the dark side, he could become another Darth Vader. Perhaps even worse…
- Jedi Academy Vol.2: Dark Apprentice
Spoiler:
NOTE: Notice how Luke recognises that a Kyp Durron wielding the dark side could become even more powerful than Darth Vader, which Kyp does, and yet he states that Exar Kun is an even greater threat than that.
He is also stated to be utilising his 'full might' at this point:
Against the full might of Kyp Durron
- Jedi Academy Vol.2: Dark Apprentice
Master Luke himself is more powerful than he had even been in Dark Empire:
When we had met before I had felt power in him, but now, after his experiences with the Emperor Reborn, his power had been redoubled.
- I, Jedi
So we have indisputable evidence that the spirit of Exar Kun is fully capable of holding two super powerhouses under his will, even from across the galaxy; whilst simultaneously ragdolling and controlling the likes of Corran Horn, Streen and others.
With this established, one must realise that this is only Exar Kun in a spiritual form. Not nearly as powerful as he is in life. Both Tenebrae and Sidious demonstrated that they were mere fractions of their former selves in spiritual states.
Spoiler:
NOTE: Marka Ragnos' spirit required the combined dark side energies of Yavin IV, Byss, Korriban and many other sites to return to only a semi-restored state. Yet Exar Kun, his canonical superior, only had the temples of Yavin IV to maintain his existence.
This makes sense, when you realise that Exar Kun is held in exceptionally high regard:
If Kyp could face this final test, Luke would know he'd passed through the fire of his testing-tempered by forces as dire and powerful as those Luke himself had endured.
- Jedi Academy Vol.3: Champions of the Force
There in front of him, starkly visible against the blackness of space, he could see the faint images of Emperor Palpatine and Exar Kun, two of the greatest focal points of the dark side he’d ever had to face. They were standing there before him, gazing back at him. And laughing.
- Specter of the Past
Concise, I hope.
TL;DR: Exar Kun >> Spirit of Exar Kun > Master Luke & Dark Lord Kyp > Darth Plagueis.