Wow! After a long time is great to see the covert art working again on Airfoil and AppleTV. I'm trying it now on version 188.8.131.52 on OSX El Capitan and is working great.
I've noticed that the cover art resolution issue happens only on some covers. Other's show much better resolution. I think this is simple that some are stored on Spotify data base with different resolutions and the effect is seen when watching the covers on big screens like the AppleTV. Check for example the song "Memories of You" by "Carol Weisman" it looks nice even on the AppleTV.