I quit WoW in Jan. 2012, but prior to that I was a hardcore raider for 7 years. I was my guild's primary Warlock and also specialty Warrior Tank. Here are some of my guild's accomplishments:
Our WoW Achievements:
• TBC: Pre 3.0 - Everything up to and including Mu’ru. Kil’Jaeden Post 3.0
• Heroic: Glory of the Raider and Immortal - Pre 3.1
• Heroic: Glory of the Ulduar Raider - US 76th
• Heroic: Iron Council - World 7th, US 3rd
• Heroic: Observed (Algalon) - US 56th
• Heroic: He Feeds on Your Tears (Algalon, No Deaths) - World 12th, U.S. 6th
• Heroic: Alone in the Darkness (Zero Keeper Yogg-Saron) - US 48th
• Heroic: A Tribute to Insanity - US 55th
• Heroic: A Tribute to Immortality (ToGC 25, No Deaths, Achieved Twice)
• 10 Man Heroic Lich King - US 12th
• 25 Man Heroic Lich King - US 26th
• 25 Man Heroic Halion - US 17th
• Attained #1 US ICC HM 25 Time Attack during 3.3.3
• Guild Grand Marshals Achievement Season 9.
• RBG Season 9 - 25US, 3rd on Rampage
• All of our Rank 1 Gladiators.
Final Tier 25 Man Rankings:
• Tier 13 - Ceased 25Man raiding at 5/8 64 US.
• Tier 12 - 69 US
• Tier 11 - 48 US (39th to finish 13/13H)
• Tier 10 - 26 US
• Tier 09 - 38 US
• Tier 08 - 51 US
• Tier 07 - 120 US
• Tier SP - 500-600 US
Based on my WoW experience, I would say that the type of monitor you use should depend on what type of player you are.
Some players use a lot of realtime data readout mods and can make legitimate performance improvements if given more pixel real estate. A larger screen without extra pixels just wont do for that type of player. The font text cant get any smaller without distortion. The goal isn't to acquire more mod windows covering the viewing area, but rather to open up the viewing area so the player can respond to a wider range of visual cues. Hardcore raiders with output maximizing or resource managing related tasks tend to be this type of player. Of course expect your framerate performance to drop by approximately 45% which switching from a 1920x1080 to a 2560x1440.
Some players are more reactive to the 3d visual effects. These players can make more use out of smoother motion and reduction in ghosting on faster refresh monitors. Tanks, casual raiders, and competitive PvPers tend to be this type of player, although it is debatable how much the first two would gain from a faster refresh monitor. They will have to deal with more noticeable screen tearing with v-sync off. If you play WoW with v-sync on, this entire post will be a waste of time.
Keep in mind that 20fps will only give your eyes fresh(only fresh if v-sync is off) information every 50ms on average. This number isn't too problematic for most WoW players. At that framerate, you still react to movement instead of individual frames, so the 50ms isn't fully added to your reaction time. If you go below 20fps, then you start having response time issues due to non-fluid motion. The lower the framerate, the more jarring the screen tearing is. When I played, I would make graphics adjustments if my fps would drop below 60 at all. I also kept a framerate cap at 125fps. I use a TN type monitor at 1920x1200x60hz. There are many graphical options that usually directly improve playing performance. One example is how shadows help the player determine a 3d position of an object in relation to the ground. My point being that while you can turn down graphics to improve framerate, there is a threshold where this adjustment becomes counter-productive to WoW player performance.
When considering how to improve input lag in WoW, there are a few things that should be a given. The monitor needs to be TN or a type with similarly low latency. V-Sync needs to be off. Mouse cursor movement cant be processed ('smoothing'). The mouse (and keyboard) hardware itself needs to be low response time. Your network options need to be configured for low latency. Once all these local latency factors are set right and added to the much
more massive internet latency, differences in your monitor refresh rate or gpu framerate (if not critically low fps) will only be a very very small factor in total reaction time to server events. How refresh or frame rates will help however, is through eye fatigue and better motion tracking. You can make better reactions for longer.
If you run mod heavy and use cpu gobbling graphical options, having 4 overclocked hyperthreaded cpu cores with process affinities set might be more important that getting a 120hz monitor and might be a necessity before getting a 2560x1440 monitor. WoW runs extremely cpu heavy in a raiding environment.