Just a few precisions : the diversion problem appears when I install the drivers from testing repositories (which is my priority). I uninstalled them and reinstalled the .deb packages generated by the official AMD/ATI binary and saw in the log that the diversion worked well. I kept my xorg.conf and everything worked fine (glxgears, fgl_gears and WoW) but i did not see any performance improvement.
So I installed back the testing repo drivers and got the error again.
When you say "re-edit your worg.conf again" what do you mean? I mean, in both cases (testing repo or official binary) the xorg.conf I copied/pasted earlier should work the same? Shouldn't it?