you will notice much more the size of the video than the resolution.... And you may notice the black bars too.
I don't know how big your TV is, but keep in mind than in an 12.9 iPad the actual size of a 16:9 video is slightly under 12in, and it's even smaller if the video is wider like a lot of movies or some YouTube videos made to better fit current smartphones. For reference, a 16.9 video is 9in in a 9.7in iPad, 9.7in in a 10.5in iPad and 10.5in in an 11in iPad...
At this size anything above 1080p is barely noticeable unless you look for it...