Alex from the tests I did 3.13 doesn't seem to behave the way you described.

With power state set to "high", desktop effect OFF and no glxgears:
MPEG DECODING (1920x1080): 5 frames/s
MPEG DECODING (1280x720): 21 frames/s
H264 DECODING (1920x1080): 8 frames/s
H264 DECODING (1280x720): 5 frames/s

With power state set to "high", desktop effect OFF and glxgears:
MPEG DECODING (1920x1080): 77 frames/s
MPEG DECODING (1280x720): 117 frames/s
H264 DECODING (1920x1080): 16 frames/s
H264 DECODING (1280x720): 91 frames/s

With power state set to "high", desktop effect ON and glxgears:
MPEG DECODING (1920x1080): 77 frames/s
MPEG DECODING (1280x720): 117 frames/s
H264 DECODING (1920x1080): 51 frames/s
H264 DECODING (1280x720): 91 frames/s

It seems even glxgears wasn't able to keep highest power state in all tests:
enabling desktop effects was enough to keep higher state in the second-last
test.

What do you mean by "plain video playback"? If you mean without using vdpau it
works flawlessly.