How do I uninstall StarCraft II on OS X?

The standard procedure is just moving the Starcraft II folder from Applications to the Trash. Your save-games,replays etc. are stored in ~/Library/Application Support/Blizzard/Starcraft II/. If you keep that folder, your save-games should still be there after a re-install. I haven't tested this though, so I suggest you make a backup copy of that folder before you perform a fresh install.