Don't know if this is due to compression artifacts (even if I don't think so, because 20Mbps should be enough to avoid them), but it seems that the superresolution algorhythm you are using is not that effective... a simple avisynth bicubic upscale, using the compressed MKV SD video you provided, sharpened a bit, seems very similar...
http://screenshotcomparison.com/comparison/117604
what do you think?