The ID84 looks like a step backwards for video playback compared (on paper) with the ID41. Probably not as good for Mythtv, though I don't know of any proper head-to-head real-life comparisons.

I also have an AMD E-450 based system, and it is nowhere near the ID41 for video playback.

Note that the vast majority of video content we watch here comes from the UK. We do have stuff from other sources too, though, and the ID41 plays it all.