One simple passion metric is just the average number of artist plays per listener. Presumably if an artist’s listeners are playing an artist’s songs more than average they are more passionate about the artist.
The results are interesting... run against a very large pool of artists, the ones who rank highest on the passion index are ridiculously obscure. When the scope is limited to the 1,000 most popular artists, Nine Inch Nails ranks #8, just above Opeth and right below Porcupine tree. Limiting the scope to the 100 most popular artists, the 5 groups with the most passionate listeners are (in order) The Beatles, Radiohead, Nine Inch Nails, Metallica and Pink Floyd. Interesting stuff.