Been tweaking and playing with the FireTV running the MythTV plugin. After working out some config issues, it does work.
Problem is, any major panning is excessively jerky. Any excessive character movement is excessively jerky. I went down to the Chromebox and it was smooth. Next test is to swap the devices around and run the Chromebox over wifi and the FireTV over hard wire, but looking at CPU stats, I think the jerkies are coming when the CPU pegs at 100, so I am fairly sure the FireTV is not up to the task. That said, it is ok watching news in the morning, but anything else is just too jerky to enjoy.
I did notice that the Chromebox CPU got fairly high, so it might be worth getting an i3 NUC, but the pricepoint there is a bit too high for my taste. The Chromebox does seem just fine, so that may be my future 'dvr' solution for the two main TVs. The bedroom will probably stay with the FireTV. I may occasionally view DVR content from it, but not the bulk of it.