Normal and Heroic more taxing on your hardware than LFR..? What are you talking about? You are subjecting your hardware to the same things, there are 25 players throwing particles around, which is the environment that puts most pressure on your PC in this game.
Largely because different things happen in LFR as compared to normal and heroic in the background contributing to lag, which is fairly common knowledge.
Considering that I am a person who raids heroic 25-mans, I think I know what I am talking about.
And as for giving a proper answer to the OP, you will not run the game at 80 FPS in 25 man raids with that card, though it will still run quite well, the 660 is quite good for it's price.
... isn't that basically what I said with my first sentence? :s
Clearly Danellos, you do not know what you are talking about regarding this subject.
We are talking about the pressure being put on hardware here, of course there are major differences between LFR - Normal and lastly Heroic modes, but they are things like difficulty, mechanic's and loot quality (and of course some bosses/phases only being available on these difficulties). As for the amount of pressure being put on your hardware, it is the same across all three modes (your computer does not suddenly start crapping itself because of these things, it couldn't care less, what it cares about are things like the amount of particles flying about, amount of players in your vicinity etc).
The difference is only in if you are doing 25 man or 10 men. I have no idea where you got the idea that Normal and Heroic modes put more pressure on your hardware, what is your reasoning behind this claim?? You raiding either Normal or Heroic has nothing to do with this, you do not seem to understand why said scenarios are taxing on hardware, which is the problem here.
Do players participating in LFR not cast as many spells, and thus do not generate the same amount of particles?? Do they use less addons? Please explain your thoughts, simply saying "which is fairly common knowledge." is simply ridiculous. As is your statement "Considering that I am a person who raids heroic 25-mans, I think I know what I am talking about." as though it gives your argument some kind of weight that you raid Heroic modes, it does not in this case.
If you do LFR I guarantee that you will get the same performance as you get when raiding Heroic. This is of course if everyone in the raid is actually participating, if there are afkers doing nothing then that can of course increase your fps in LFR, but I was speaking from the viewpoint of everyone being equally active as if they were doing Normal or Heroic (I realize that this might be wishful thinking) .
As for the other thing you said, I said "as for giving a proper answer to the OP" because you claimed that he would not get the same performance in Normal or Heroic mode, as he would in LFR, this is simply false. It is not possible for him to get 80 fps on Ultra in any of the difficulties, it is not more "reasonable" to believe that he would get 80 fps in LFR, as you claimed.
After that I simply answered his question, and we both agree that he will not achieve such a high fps with said hardware.