First of all, whilst that list is pretty impressive, it can't be used to measure their power. For example, Darth Bane is nowhere near as powerful as Darth Sidious is. It's merely stating that some of the most powerful Sith are human. Though if we do wish to look at accolades that pertain to the Sith Lords, these are fairly favorable to Exar Kun:
Then, abruptly, he stiffened, his breath seeming to freeze in his throat. 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.
- Specters of the Past
This would seem to indicate that out of all things strong with the dark side, Exar Kun and the Emperor were two of the strongest. But I wouldn't use this against Vader, simply because it isn't definitive.
Now it has been claimed by others that the feat in question, that of the Corsair, is a feat that is ambiguous in its nature. Abel G. Pena and Tom Veitch however, seem to disagree:
Q: In the Knights of the Old Republic Campaign Guide, under the Sith Battleship description it states that Exar Kun recovered his ship the ‘Corsair’ in the same way Darth Nihilus recovered the ‘Ravager’ on Malachor V. Does this mean that Exar Kun summoned the ship via the Force from the depths of the Massassi Temple of Sacrifice?Peña: That said, looking over the text, it appears to me that a direct analogy with the Ravager and that your interpretation is essentially correct. (Another example, though in reverse, is Emperor Palpatine using the Force to bury the Lusankya, in plain sight, beneath the surface of the planet Coruscant.)
Even if you take prep or a nexus into context, Exar Kun grew vastly more powerful after said feat. Thus such a boon would be irrelevant with this incarnation.
In terms of the Starkiller feat, the issue here is that there is a clear difference in the mind of Starkiller himself. Thus I'm not at all convinced he'd bring such potency to his fight with Vader, nor that he can really summon strength such as this consistently in the fight:
"Starkiller closed his eyes. He didn't have time to wonder what was going through her mind. He had to think of something fast, or Juno was going to die.There was only one thing he could do, and although he knew he wasn't likely to survive, he didn't hesitate. What was death when the love of his former life was at stake? Besides, anything was possible. Dying, as he had thought once before, always seemed to bring out the best in him.
With his mind and all the power of the Force, Starkiller embraced what remained of the frigate beneath him-and blew it into a billion pieces."
- The Force Unleashed II
Now the problem with your 80% of Sidious quote, is that in reality, he not only says his powers are curbed now, meaning growth would be negligible, and given the source, that'd be binding Word of God Lucas Canon. In that, it'd contradict everything that claims growth in your following quotes. But more importantly, we have no idea what that means into relation to other characters. For example, does that make Yoda 90%? It's not really usable, in all fairness.
As far as knowledge of the Force goes, Vader really can't stack up to the most powerful Sith holocron ever:
Locked within it's recesses, accessible only to a Dark Lord of the Sith, are the forgotten histories and lore, dating back a hundred thousand years and more.
- Odan-Urr, Tales of the Jedi
The most powerful Sith Holocron contained Sith teachings and histories that covered some hundred thousand years...
- The New Essential Guide to Weapons & Technology
Which Kun canonically got more powerful because of:
Exar Kun might not have risen to power so quickly had Odan-Urr destroyed the Sith Holocron when he’d had the chance.
- Jedi VS. Sith: The Essential Guide to the Force
Odan-Urr kept this Sith Holocron for nearly a millennium before it was stolen by the Jedi Exar Kun, who used it to learn many secrets of the Sith.
- Jedi VS. Sith: The Essential Guide to the Force
Odan-Urr goes to the Force and the darkest power in the galaxy walks away with something that will make him even stronger.
- Tales of the Jedi Omnibus: Volume 2
That's besides the Great Library on Ossus; which gets ruined by supernovae immediately afterwards, where he raids the Chamber of Antiquities; the most precious artifacts in the entire Order at the time:
Looking at all the precious artifacts his Massassi have stolen, Exar Kun knows he has more wealth and knowledge than he could ever use.
- Star Wars Omnibus: Tales of the Jedi Volume #2
Exar Kun didn't tank supernova blasts. But he did tank an attack from Ood Bnar, who survived being on the surface of Ossus as it was bombarded and devastated by ten successive supernova waves. Yet despite using his full power, he couldn't injure Exar Kun.
As for Vader's durability feats, they are quite impressive. But it's honestly not really beyond the norm of Kun's; Nadd unleashed 'cataclysmic' energy and collapsed boulders on-top of Kun:
Kun survives being broken in a hundred places and remains entirely capable of maintaining his mental faculties and speaking. Not beyond Vader's durability but it's a strong indication of what even Jedi Exar Kun is capable of surviving. Beyond that, we have seen the likes of Darth Traya, the Exile and her party recover from the most greivous wounds. Including even self-impalement upon a lightsaber. Yet they're effectively fodder in a contest with a great Dark Lord such as Exar Kun.
Now, I won't go into depth about Force lightning, because I'd prefer to stick to Kun's living feats. I will however say that suited Darth Vader has never been close to Darth Sidious, let alone surpassing his ROTS incarnation. Let's take George Lucas into account:
"He tries it with anybody he thinks might be more powerful, which is what the Emperor was looking for in the first place: somebody who would be more powerful than he was and could help him rule the universe. But Obi-Wan screwed that up by cutting of his arms and legs and burning him up. From then on, he wasn't as strong as the Emperor—he was like Darth Maul or Count Dooku."
- George Lucas, Rolling Stone 975
So no, Vader after Mustafar was never his Master's equal. He's arguably approaching Plagueis, but Sidious is an entirely higher league.
As for returning from death, one of Exar Kun's cronies became the master of such a constant state of willing himself back to life and living as a corpse through sheer rage. He went by the name of the Lord of pain and wasn't really very powerful at all given his peers.
Exar Kun on Yavin IV was already a great power, enough to replace Freedon Nadd's spirit. Yet Nadd had managed to empower Ommin to the point of causing mass rage in a city, maintaining dominance on Arca Jeth. Nadd was also responsible for corrupting Onderon to an unnaturally strong dark side nexus. One-shotting Vodo across the galaxy and more. Kun's power grows immediately after this, and then unlocks more power as he masters Alchemy, becoming an 'extremely powerful figure'. This is prior to leaving for Cinnagar, where he stalemates Ulic and then spends several years gaining 'tremendous' dark side power and obtaining the aforementioned Dark Holocron which only increased the rate by which he grew in power. Until he became far more powerful than any other Jedi; even Ulic, who had once been his match.
This scales him massively above his Jedi self, where he has the augmented feats of causing intense lightsaber clashes; which could be heard from kilometers away, breaking through a multi-feet deep wall of sheer Mandalorian Iron; material even used in some capital ship hulls.
This all neglects to mention his in-combat Sith sorcery techniques; and his Force blasts, to boot.