With large groups, CPU is key, as well as the internet connection, and server lag. It's hard to tell any difference in WoW at 40 vs 50 or 60 fps, only when it drops into the 30s and 20s. 40 is pretty smooth.
I am running i5 2500k with 560 ti, and I am getting 45-55 fps at 1080p in 25 man raids. It's not bad at all. It's the game engine that's very old (they started creating it back in 1999, it's almost 13 years already!).