Catalyst 14.4 Has Advantages Over Linux 3.15 + Mesa 10.3 Git
With the current state of Linux 3.15 and Mesa 10.3-devel out-of-the-box for these HD 5000/6000 series graphics cards, there's still a way to go to compete with Catalyst 14.4 unless taking additional measures: environment variable tweaks, out-of-tree patches, etc.