After much troubleshooting (thanks @Kronos), I was able to get resolutions >1680x1050 working by reverting to the previous version of the Radeon device driver (52.36). With 52.37 I can't go above 1680x1050 no matter what I do. There are issues with this particular card going above 230MHz, but I am now running stable 2560x1440 @ 57Hz.