How to make sure album art shows up everywhere (Mac & iPhone) while using iTunes Match?
You don't need to download a song on your iOS devise via iTunes Match to get its artwork updated on that device. All you need to do is go into Settings -> Music -> turn off iTunes Match, and then turn it back on. All artwork will update over a few minutes. (I turned device off and back on again before turning Match back on just to be safe, but not sure it's necessary.) 

You can go into your Music and see the little spinning wheel constantly working a half second at a time (for each album cover being updated from A to Z). with iTunes recent update to artwork, this is how I got the new artwork over to my iOS device.