I actually tried WoW yesterday (latest patch) for the first time on my 5K iMac and was surprised the M290X could run it very smoothly in 5K resolution. I didn't expect it to run well at anything higher than 1440p, but there I was, running through Draenor in 5K without any lag or stutters, with AA turned on and settings set to high. The game almost looks weird in this resolution, to be honest. Oh, and I played the OS X version, not in bootcamp. I didn't test it in over-populated areas or raids, though, but everything I tried was really smooth.
I can only imagine what M295X can do, but the M290X is surprising me more and more every day.
Funny how you can play a game in 5K with no problems (though, admittedly, WoW is not super-demanding) and still have Mission Control lag
May I ask you to run WoW with the settings that I use in the video I posted above ? (5K all ultra except shadow and water to the minimum)