Is a Graduate Performance Diploma considered an academic degree for purposes of PhD admissions or faculty job search?
I did my undergraduate in a large, prestigious music school, which also had (probably still has) a Diploma option.  Before I answer your question, let me tell you something about my degree requirements.

I had to take lots and lots of credits in my performance instrument, and ensembles, large and small.  Then there was lots of music theory, music history, music literature.  A tiny proportion was in other departments -- I took one English, one math, one history, one science, and one social science course, if I remember right.  That's not much for a 4-year degree!  But now, for the Diploma, take out everything except the performance instrument and the ensembles.  Well, apparently at Johns Hopkins, you also do some music theory-type courses as well.

The students that I saw doing the Diploma were typically from another country, were planning to go back after the Diploma, and were there just to further their study of their instrument with a particular teacher, usually a famous name.  It was kind of like a post-doc.  They were soaking up the great person's approach to technique, expanding their repertoire, getting more experience, polishing their performing.

Music schools offer the Diploma because they know there are students who only want to work on their performance instrument.  So they cut out all the extraneous stuff and give the person a Diploma.

This would be good preparation for a performance career (orchestra, chamber music, solo concertizing), and for teaching the instrument in a private studio.  However, it would NOT prepare one for teaching in a public school, community college, college, or university.  However, it would help one prepare for teaching in a conservatory (if the person were a strong enough player -- s/he would have to be extremely strong for this).

If you are from another country and are planning to go back, chances are you are going to do, or you did, some academic work in your home country, and that will fit better with the academic system you plan to seek a job in.

A lot of the Diploma students were astounding musicians, though, who did seem to have a shot at a well-paid performance career.  That I can't be sure of, though, because I don't know what the employment options for classical musicians are in Europe (which is where many of the Diploma students were from).