This weekend, I tried out the Elixir of Empowerment on the Gruul fight and I must say, even though I was too preoccupied with the environment of the fight to give it too much scrutiny, it did seem to possibly help. I mean, obviously it should. Taking into consideration the coming changes to how many elixirs a person can use at a time, it is not worthwhile to think about which elixir is going to help you the most. I think that the obvious thought is to flock to the one that openly states "more damage". But is that really the best choice? How does something like the Elixir of Empowerment truly pan out in comparison?
Elixir of Empowerment
Use: Decreases the magical resistances of your spell targets by 30 for 1 hour.
Blizzard has a way of muddying up the verbiage sometimes where spell benefits are concerned ~ this effect is the same as Spell Penetration you will occasionally see on gear or enchants and helps to negate the 'partial resists' your spells sometimes suffer. Whether or not you can benefit from spell penetration can be determined from what you are seeing when you cast spells at things. If you toss a fireball at a boss and anticipate for it to do 1800 damage, but only 800 damage lands, you just suffered a fairly big partial resist. To this extent, if you are using a damage increasing elixir rather than empowerment, you just tossed a big portion of the benefit your are supposed to be getting out the window.
Fire magic specifically seems to be particularly prone to this vulnerability, since a great many monsters and bosses in WoW seem to come with some measure of fire resistance. One of the hidden benefits of frost has long appeared to be the relative lack of resistance to frost most monsters have. This may change come Serpentshrine Cavern, but to date frost has always typically been pretty good in this respect. From my observations, shadow, arcane and nature magic have tended to fall somewhere in the middle.
The thing is, warlocks have less need for spell penetrating bonuses since their Curse of Shadows includes a fairly large resistance debuff ~ enough so to generally negate the need for any additional help, if they are actually using it. Curse of Elements similarly extends this benefit to mages while using fire or frost spells. (No similar debuff exists for arcane or nature, that I'm readily aware of.) Rank 4 of CoE is nearly three times more effective than the Elixir of Empowerment, just in its resistance debuff alone. Ideally one warlock should put this up on a boss fight (even though warlocks benefit less from it themselves), because it helps the raid succeed. I mean, I guess they do some fire damage too ~ shouldn't forget that.
So if warlocks can significantly decrease the need for spell penetration on a boss fight, then you shouldn't really need it, right? To some degree. You also have to keep in mind that while in a small raid instance such as Karazhan you will often have too few casters of the same type to make the make resist debuff curses seem attractive in comparison to damaging curses. (assumption corrected by David)
So basically, in any situation where you are not seeing partial resists… don't worry about it. But if a boss is shrugging off more damage than you are comfortable with, it is perhaps time to consider losing the Elixir of Major Firepower and popping that Elixir of Empowerment instead.