Well, if you reduce a monster's life expectancy by "x", it stands to reason to increase his damage output by "x", so he deals the same amount of damage in half the time. Two examples:
If I end up doing this, I wouldn't use "50%" as the "x". More likely 25% or 30% (okay, 33.33333%).Keep in mind that bumping up the damage by a great amount can have a much larger effect in a few ways.
1) Focused fire. With normal damage, a group of monsters concentrating on party member might get that party member close to death, forcing the party member to retreat, take a second wind, get some healing, etc. With 50% more damage, you might just outright kill them.
2) Weakens in combat healing. Normally a cleric healing might give you another round or two of action. With higher damage, it might not even buy you a round. Further, a standard action becomes a much bigger cost action wise when you cut the number of rounds in half.
Not saying the idea isn't good, just remember its a little more complicated then take off X here, add X there, everything is fine and wonderful.
Y'know, I like this better than reducing HP. Whatever damage the PCs tell me, I just increase by, say, 25% and apply it. Whatever damage I roll for the monsters, I add 25% and apply it.We've played a few sessions and found combat a bit on the slow side, so last time, we just doubled all damage for PCs and baddies.
Worked like a charm. Mind you, this is a high-level campaign, so I'm not sure if this is the best solution at lower levels.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.