Debian sid should already have ati/radeon 6.9.0 open source driver so the only thing you should do is to check if EXA is enabled.
Make sure that your xorg.conf has:
in the "Device" Section.
Option "AccelMethod" "EXA"
To make sure it's enabled open /var/log/Xorg.0.log and check for the following lines:
I think that's all you need to do.
(II) EXA(0): Driver registered support for the following operations:
(II) Composite (RENDER acceleration)