Although I've heard Extra Chorus was intended to be watched before Mirai Fukuin, watching it afterwards was a much better decision.
The story is fairly simple. Chapters 1 and 3 of this special deal with Mikiya and Shiki, with Chapter 1 being fairly inconsequential to watch, and Chapter 3 being, at least from what I believe, an unintentional but beautiful ending to the series proper. Chapter 2 deals with a new character, and the return of Asagami Fujino, which is very nice. Chapter 2 allows Fujino and her friend to develop very well in reaction to events from the previous movies and what they believe in
The art is, as per usual, really nice. The use of CG works very well, and there's nothing outside of the ordinary, just a very simple, solid presentation of animation by ufotable, and a very nice ending frame.
The sound... well, there's not too much in terms of sound design to notice. As for the composition, Yuki Kajiura and Kalafina are purely wonderful in their use of vocals that accentuate the mood and characters of scenes, especially the ending.
Now you've probably noticed I've said the word ending multiple times, and that's because this OVA is worth watching just for that. It really is the absolute final conclusion of the series, albeit some might consider it redundant. I personally think, because of its connection to the beginning in music, it closes the loop once and for all.