An apparently known issue for both platforms, indeed. Included should be the solution, bolded part, from http://www.rage3d.com/board/showthread.php?t=33991741...
I've found that you must restart immediately after this configuration in order for it to work.7. sudo aticonfig --initial -f
8. sudo aticonfig --vs=on
9. sudo aticonfig --set-pcs-u32=MCIL,HWUVD_H264Level51Support,1
(Optional 9b. if you have black bars normally, disable underscan (always after lightdm has stopped): sudo aticonfig --set-pcs-val=MCIL,DigitalHDTVDefaultUnderscan,0)