well, how high is your cpu load when the players are running? i don't know specifically how many percent the avivo codecs are suppose to offload to the gpu, but know that there is still going to be some cpu load. i have a radeon hd 4870 with an amd athlon 64 x2 that I have running at 3 GHz. when i play a blu-ray movie, my cpu load increases and stays between 40% and 60%, as i see it on the task manager performance tab.
are there any other programs running in the background while you are watching a movie that might help to contribute to a higher than normal cpu usage? is the video playback jittery or stuttering that makes you concerned about the higher than normal cpu usage?
another comparison fyi: recently, i was playing half life 2 episode 2 and had the task manager running on my second monitor. while playing the game, my cpu load stayed consistently about 50%. just the other day, i wanted to go to where it all started (with the game at least) and fired up half life. i ran the game as i did with hl2: ep2--with the task manager running on the second monitor. to my surprise, my cpu load stayed about 85%! i would have though that the percentages would have been reversed--hl using less cpu than hl2. i guess what i'm trying to state is that i had nothing adversely affecting my performance with a cpu load so high. in the case of your video, i really wouldn't give it much thought unless that high cpu load is causing your video to be jumpy, jerky, or stutter.