2700k with GTX 670, all ultra. ElvUI, Skada, BigWigs, WeakAuras and a few other misc addons, input lag for me usually happens at the start of the fight and during lust. It was brutal on Megeara rampages, and a few other fights this tier.
Yeah always played 10 man on Ultra, no issues, just with 25 man i had to change a lot of things, i guess with BigWigs now it should actually be ok with most things on Ultra.
But seeing how CPU's are all current, that's probably where the most improvement comes from.
some fights 60 fps, some 30 fps. just depends.
However, on 10 man I never got input lag and the FPS hit is far lower with that many less spell effects.
25 man is just another beast and no real computer can max it out at great framerate on every fight in a 25 man setting. tis the nature of an MMO.
Do you use max spell effects and shadows? It might greatly decrease the hit you get from loads of spells running around if you get those down, while it doesn't really affect the visual quality. I have 2500K @ 4,8GHz and GTX 480 @ 900/2000 and I'm still running worst at 30fps on world bosses like Sha of anger or the new Oondasta, with around 80-100 players killing it.
Finally managed this 2nite 25hc, the fight massively favours dks and rogues, war survivability especially when you have gaze in a beam just seemed awful, managed 180k with normal 4pc and 517 weeps, noticed that the ww glyph seems to let you hit adds in the beams from a ridiculous distance