It's a matter of perspective. Running a 1980x1200 monitor, the refresh will most likely be 60 or 75Hz. If the games you play are able to meet that number or more, then adding further fps possible from the gpu is meaningless to anything but a benchmark.
It's only if the gpu is limited to under refresh that OC might make for better available fps and smoother game play.
So run the card at stock values. If the game at ultra doesn't get refresh fps, then try OC. If it does get refresh, there'll be no gain from OC.