Hmm I'm sure that the CPU was at 2-3% when on the HD channel.
I think the CPU on its own could handle 1080i content maybe I could turn off hardware decode and see. Thing is my system plays back high bitrate 1080p stuff with no problems
I also have the same problems on my gaming rig with i7920 and 295gtx!
As far as I'm aware, there is no hardware decoding when using 1080i material. 1080p will be fine but apparently Microsoft didn't include 1080i DXVA acceleration.