Lightsaber ability
But of course, maybe this contest will be decided not by the strength in the Force of the contestants, but with the lightsaber skill the two Sith Lords have to offer. Kun, even in his apprentice days is already a "master swordsman" and the best lightsaber duelist Vodo has trained in 600 years. His most notable feat is, probably, his stalemate against Ulic Qel-Droma, who inspired respect even in his most notorious enemies ("Jedi Vs Sith: The Essential Guide to the Force", p.127):
Sylvar, at that point in time, absolutely hated Ulic Qel-Droma, yet, she still regarded him as the greatest Jedi of his era. And that was before he took the path to the Dark Side and became an even more formidable combatant. But before we discuss Ulic, let us have a look at Kun's weapon and style. As we all know, Kun utilized a saber stuff. Darth Maul, the other famous user of the weapon, was so kind to share his thoughts on that lightsaber design with us:
"Maul raised his own weapon, the double-bladed lightsaber, and triggered the power control. Twin lances of pure energy boiled forth, hissing and crack-ling in crimson loops that began and ended at the two flux apertures on either end of the device. Any Jedi Knight could wield a single-bladed lightsaber; only a master fighter could use the weapon first designed by the legendary Dark Lord Exar Kun millennia ago. Unless one was in perfect attunement with it, the weapon could be as deadly to the user as to the opponent." - Darth Maul: Shadow Hunter
Emphasis mine.
I'm going to assume, that this thoughts on the design spawn from Maul's thoughts rather than being information given by the narrator of the story himself. Still, Maul muses that one needs to be a master of lightsaber combat to handle the weapon, and – on top of that – it needs "perfect attunement" with it, not to become a liability for its wielder.
"Jedi vs Sith: The Essential Guide to the Force" describes the nature of the weapon an its use in combat quite clearly (p.107 / 108):
With more detail to be found in the "Book of Sith" (p.85):
While putting emphasis on the fact, that the weapon is difficult to handle, this likewise discribes how it is used. Notice that Exar Kun's style to wield the lightsaber (one-handed, fast) is fundamentally different from the style described here. In addition, Kun can adjust the length and intensity of the weapon on the fly, making him very possibly the least predictable opponent one can face in a lightsaber duel. And since the particular points have been argued here, let me leave my 2 cents about Kun's strength and speed:
Strength
Even at the beginning of his career, Exar Kun seemed to have been rather strong:
I don't know how many lightsaber duels you have seen, in which the opponents clashed with such brutal force, that the sound could have been heared over the distance of kilometers, but Sylvar and Exar can definitely do the job. And their duel ends in a rather special fashion:
Notice that Sylvar is strong enough to put her claws through Exar Kun's helmet, shattering it in the process. And then Kun, empowered by his anger, physically overpowers her. Kun's next "strength feat" would be his stalemate with Ulic Qel-Droma, who is quite the powerhouse in terms of physical strength. Here we have a picture of three Jedi trying to hold Ulic down, which gives them some difficulty.
This is quite impressive for Ulic's part on his own. However, one of those Jedi (the middle one), happens to be Qrrrl Toq, who is quite the extraordinary type when it comes to physical strength:
So the guy can just press five human males to a wall, while those are leaving the ground (see feet in the picture) with a rather large piece of metal – one-handed, while not particularly caring about what he does, casually cutting them down with the lightsaber in his other hand. Clearly some superhuman demonstration of strength. Not that Ulic himself was incapable of similary feats on his own:
Tossing a Mandalorian in full armor around like a ragdoll, using only one hand? No problem for Ulic. The last time I've seen somebody overpowering people that way, it was Darth Vader, aided by his mechanical limbs. Of course, Kun appears to be rather indimidating physically as well, given his "imposing figure" (see above) and his own demonstration of strength:
Destroying a Sith amulet with his fists might not seem that impressive, unless you think about the fact, that those things are crafted to ensure the survival of the knowledge stored within. It's also rather telling that Kun would "sacrifice" such an object. He probably wouldn't have done so if it had to offer new knowledge for him.
And then, there is this little incident here:
Kun, apparently, puts his fingers through the skull of the Chancellor and then lifts him up into the air, using him as some sort of hand-puppet, while the weight of the alien being sustained by his fingers alone. Not too shabby, even if it isn't compareably to the feats above.