How about GTKperf Lines test ? I had terrible performance (and 2D performance overall) with R9 270X and the latest git code for xf86-video-ati and glamor... Scrolling in firefox ie with some heavy pages results in a - what could be best described as - drag...
Here is the relevant bug report...
GtkPerf 0.40 - Starting testing: Mon Nov 4 22:24:40 2013
GtkEntry - time: 0.02
GtkComboBox - time: 0.30
GtkComboBoxEntry - time: 0.31
GtkSpinButton - time: 0.22
GtkProgressBar - time: 0.24
GtkToggleButton - time: 0.16
GtkCheckButton - time: 0.13
GtkRadioButton - time: 0.26
GtkTextView - Add text - time: 0.14
GtkTextView - Scroll - time: 0.09
GtkDrawingArea - Lines - time: 259.08
GtkDrawingArea - Circles - time: 34.65
GtkDrawingArea - Text - time: 0.55
GtkDrawingArea - Pixbufs - time: 0.38
Total time: 296.51
Here are the results. Try this with Color Tiling disabled and your in for a week of running the test (singular) I still don't get it how I got even better results than you did in all the GtkPerf tests that you posted. As regarding glitches, even when I scroll this very same page, the green bars on the forum breaks diagonally, I get some font corruption from time to time (but this could be an issue outside the scope of ati ddx driver since I get this on Sandy's IGP as well).
3D performance is great however...