Video playback of almost any typical 10Mbps 1080p BDRip is choppy (many dropped frames) in VLC if R600g driver is used instead of fglrx on Radeon HD 6620G GPU. 
Enabling/disabling composting doesn't make difference (so this issue is not duplicate of https://bugs.freedesktop.org/show_bug.cgi?id=47776 ).
Enabling/disabling "Suspend desktop effects for fullscreen windows" KWin options doesn't make difference for fullscreen playback.
There is little difference between xv and OpenGL (GLX) output in VLC - xv output a little faster but gives choppy playback too.

Issue is not reproducible:
1. in mplayer (SMPlayer is used) with R600g driver,
2. in Firefox (1080p VP8 playback) in Chrome (1080p H.264 playback) and in Adobe Flash 11.5 plugin (1080p H.264 too) with R600g driver.
3. in VLC with fglrx driver (hardware decoder is not enabled).

Issue happen only in VLC&R600g combination.

VLC always uses indirect rendering instead of direct so I check mplayer playback with disabled direct rendering and with enabled direct rendering but I doesn't notice any difference. AFAIK mplayer and VLC drops frames very differently so I check mplayer with disabled frame drop and with enabled frame drop but (again) doesn't notice any difference. So I doesn't sure what exactly is wrong in VLC&R600g combination but choppy video playback happen only in this combination for sure. In my opinion issues most likely is relevant only for APU hardware.

Video sample used for testing: http://www.multiupload.nl/56CCVTW42H (most noticeable slowdown happen since 00:32).

Software:
Linux 3.8rc1
libdrm 2.4.40+git20121123.171666e4
Mesa 9.1~git20121222.a585b8f3
xserver-xorg-video-radeon 7.0.99+git20121207.793e1b0e
Xserver 1.13.0.902+git20121207
VLC 2.0.5 (default settings tested too - no difference)
mplayer 1.0~rc4.dfsg1+svn34540
Kubuntu 12.10 x86_64, Xorg Edgers PPA enabled.

Hardware:
A8-3500M with Radeon HD 6620G.