His original body would have been able to handle it because it is already used to the power inside it. The clone bodies deteriorated because they had no time to adjust to the power within them.
Episode III indicates otherwise, it appears intense usage of Force Lightning for instance causes the wielder to damage himself/herself. If you'll also notice with Darkside corruption, you begin to see deterioration of one's face in Old Republic as well as KotOR. The original body holds up better, I'll grant you that, but I suspect the Emperor (pre-clone) didn't use his powers to that level because there was a serious price that he would pay for doing so, a price that would seriously debilitate him, perhaps fatally.
Clones that used the lightside of the Force appear to not suffer the same ill effects, it seems as though one damages one-self with intense usage of the darkside.
Actually Revan could have beaten Malak easily. Malak was never stronger than Revan. Malak even said so himself. This is a mis-match for one reason. The Rule of Two. Vader is one thousand years ahead of Revan. And since the plan worked, Vader is obviously more powerful than Revan.
A battle isn't solely measured in strength, while Vader may have more raw power, counting Revan out is a serious mistake.
If you've ever played Star Wars CCG by Decipher there is an interesting comparison you can make, okay Vader would be on average a power 6 card (he has an ability 6 since he is a Dark Jedi), Revan (card doesn't exist but I think we could get a rough estimate), would probably be a power 5 ability 7 (he was a Sith Lord (the master not the apprentice), bare minimum he's ability 6). While this would give vader a +1 power advantage (Vader is also immune to attrition < 5), Revan's power isn't that far behind Vader's (since I'm guessing Revan would have ability 7 (like Palpatine or any Jedi Master, there is a good chance he's immune to attrition); the deciding factor would be battle destiny. If you have a total ability of 4 or more you draw 1 battle destiny, both sides thus draw a battle destiny.
Now with battle destiny being thrown into this Revan has a few ways to win.
1. Draws a battle destiny that is +2 over Vader's draw (so Vader's side loses 1 life force, if Revan draws +1 over Vader's then it's a draw, but Revan may still win.
2. Vader draws a higher battle destiny or equal battle destiny, but Revan drew a battle destiny of 5, Revan's side loses some life force, but Vader has to be placed in the lost pile because there is an attrition of 5. Revan on the other hand is immune to attrition so doesn't have to be forfitted.
Didn't bother throwing lightsabers into it because they'd probably both kill each other with the sabers rendering it a moot point.
To summarize what I'm trying to say, Power isn't everything, there is also other factors that must be considered.
Still, the damage Kun could do as a spirit was very good. Not to many Sith can live as a spirit for millenia and then attack the Jedi Order.
He might actually have been more powerful as a ghost than when he was alive, remember he stole the life force of hundreds if not thousands in order to become that ghost. So considering we're dealing with a flesh and blood Exar Kun, I don't think Skywalker would lose to him. Also he beat Luke Skywalker (whom was only a fledgling Jedi Master at the time), using the force potential of Kyp Durron to augment his attacks, there is a significant chance that without Kyp Durron's potential being used, Skywalker might have been able to take out that force ghost all on his own.