We call it:
Killing Blow
Prerequisites:
Power Attack
Weapon Focus in Selected Weapon
BAB +6
Everything else is the same. Improved crit and Killing blow, with a Keen, Bursting weapon is nasty, but you have to be 8th level (min) to get these feats, and the extra damage is not unbalanced since that is the level where you get to the instant death spells. Here, you just take lots of damage on a lucky hit. Instant death and you die if you fail a save, and it is easy to choose the correct save and augment the DC enough where any opponent is going to be sweating the roll. Phantasmal Killer v Fighters, Finger of Death v Wizards, etc.
Increasing damage possibilities is almost necessary to allow fighting classes to keep up with the spellcasting levels starting about the level where you can get Killing Blow and Improved Critical.
Where it gets disgusting is a strong fighter, with both feats and a highly enchanted item, who has a lot of attacks. Then the likelyhood of a crit happening every round is a lot higher, especially when you add in relatively low level spells like Rapid Strike and Haste to the equation.
However, they are still not as deadly as spells that are achievable at lower levels on a shot by shot basis, or even some of the more potent poisons out there.
We find it to be strong, but not unbalanced, even when maxed out in its effects. It does have a string of feats to achieve and another feat and a magic weapon to get the highest effects. Not too bad, but not too strong.