I'm not an expert here, but I think by going to full screen and watching movies at a higher resolution than they were intended you start noticing the artifacts in mpeg2. I could be completely off base ...
1920x1080@480Hz is the logical limit considering it's the same bandwidth as 3840x2160@120Hz (4k) which is the limit of a DP1.4 cable. I expect they'll try to push the technology further when HDMI 2.1 ...