I also can not test your performance because i do only have a hd5670 and not a hd5870
and openCL in the radeon opensource r600g isn't ready yet for testing sorry.
and setup?? what do you mean? i use 2 stock kubuntu setup and oibaf ppa or xorg-edgers setup
what do you think about monitor support? i test VGA and DVI and HDMI single monitor support i can not check multimonitor support.
2D usage the only known bug for me its the flash white control player panel bug... this makes flash control panels hard to use because the pannel goes white. this is maybe a transparency bug i save a similar bug in hon if the hero cronos makes a time stop shild it goes white to..... . but i think this is a oibaf ppa bug because the normal stable radeon do not have this bug.
3D bugs the stable radeon do not have many bugs if you use Git /unstable PPAs you do have bugs sometimes right now i do have some texture bugs in hon... i think its the new GLSL to galium3D translator/compiler.