In all seriousness that is overkill, and not needed for WoW...
WoW is not a graphical intensive game, compared to other games (like C3 for example)
and the fact your friend has 2 780's in SLI (regardless if WoW supports or not) is overkill.
But to answer your question 1440p is super awesome, and will show a truly superb picture quality. my only Beef is, WoW is not the game for that setup for that particular eye candy.
on the point if it will improve when he plays it competitively, I would say no, only thing(s) that are really going to change is the overall picture quality, the fps won't change, graphics in-game are already the highest they can possibly go (due to 780's), sure its a sharper picture, and the colors are more defined, but I don't see how that is going to make him better. he can already tell exactly what spell his opponent is casting.
Sounds like money to burn to me, but hey if that's what he wants to do; ahem brother!
But after all, this is OCN!