I don't know about the WTF files etc but he shouldn't need to do that, if KC is set to 1920x1080 then regardless of what WoW said, it should load ip in the maximizer as 1920x1080 or atleast that's how it's always worked for me.
Maybe the second monitor isn't set @1920x1080 therefore KC and WoW can't set it at that res?