With something that heavy, I would also say the wielder should take a penalty to CMB and CMD because your center of balance is going to be thrown off quite a bit.
It should also impart a penalty to certain skills, such as Climb, Acrobatics, Ride, and Swim, probably Stealth and Fly as well, so basically everything that armor check penalties apply to. Unlike armor, which has its weight fairly evenly distributed across the whole of the body, this thing will have its weight concentrated in one spot on the body.
On the flip side, I could see it gaining the wielder a bonus to something like Intimidate, possibly Diplomacy and Perform in certain circumstances.
Another thing to consider, even if someone is sufficiently strong enough to hold and use a weapon of that weight, it won't be easy, or comfortable, and will drain the person's stamina. So perhaps they're always fatigued, or become so after a battle or any strenuous activity as long as they've been wearing it or using it.
I could also see them taking a hit to their land speed, killing all of their swim, climb, and fly speeds (if they have them).
Since that level of weight is going to alter the balance of the weapon any weapon that heavy should be considered an exotic weapon. There is a huge difference between wielding a balsa wood sword and one made out of metal. What you are suggesting is as different if not more so than that. This would mean that feats specific to the normal weapon like weapon focus do not work with the altered weapon. You would need to take it separately for the altered version of the weapon.
It should increase the damage, but no by the same multiplier as the weight. Much of the damage of the weapon comes from the amount of force it is swung with. This would be determined by the STR of the wielder. If the wielder has sufficient STR to wield the weapon they are already getting a larger bonus. But also keep in mind that being strong enough to pick up the weapon is not necessarily strong enough to wield it. Someone using an 800 pound sword is going to need a very high STR to use it.
The character would need a minimum Strength score of 26 just to be able to carry the thing and be able to carry even a single trail ration, and that would be their maximum heavy load.
Or they'd need a minimum of 29 Strength to do the same thing at maximum medium load. To have it as a maximum light load, the character is going to need something like a 34 Strength (though that's only a rough guess on my part).
I doubt even Guts has a strength score that high, and his dragon slayer 'sword' might weigh in around that number.
As it weighs four times as much as its "wielder", the poor chap won't be able to swing it regardless of strength as he'll just tip over. He'll need a scabbard and belt made of kevlar. Even walking will be incredibly awkward. And be careful where you sit. Conversely, it should do immense damage if it connects at usual weapon speeds, because there's so much momentum going on.
RL physics aside, if it weighs 100x as much as a normal sword, you'd need Str in the low 40s to wield it (+5 Str = double load capacity, so x100 = +33.2, so that's 43 Str). It might have 5x the dice of a normal weapon, or even more.