Last month my fiancé and I bought ourselves a new 65" Sony Bravia 4k UHD TV for our shared birthday.
Watching live football in HD looks like crap on Charter TV HD. Found some great suggestions on reddit and other websites to improve the picture. Significant improvement but still not great while a play is going on with all the movement on the field.
In hopes that the Android TV feature could download the Spectrum App to stream to see if it's better. Unfortunately the app isn't compatible with my XBR-65X850D! Dammit!
UPDATE Came back to writing this after trying some things. The picture sucks on my 47" in the bedroom too. Looks like it's a Charter broadcast issue not my TV.
I have FIOS and can echo your experience with an 810c. Cable just generally sucks on a 4K TV. Some channels are worse than others, but 4K really brings out the compression artifacts in broadcast TV, especially at 1080i. For football, the best I've found is using DirecTV Sunday Ticket through the Xbox One (which doesn't help you for a variety of reasons) or streaming the game through....less than legal methods.
It's not just you, this is what happens when the broadcast networks are too stubborn to update to 1080p. The bandwidth limitations are almost nonexistent, 1080i is useless. My personal rant aside, try bumping it down to 720p instead of 1080i. You'll at least remove the interlaced conversion from the equation, and go straight from 720p -> 2160p, as opposed to 1080i -> 1080p -> 2160p.
Edit: And a little tip, if you can deal with the brightness drop, enable Motionflow Clear when watching sports. It significantly reduces motion blur at the cost of picture brightness.