We need to recognise that the distinctions Composer -> Performer -> Listeners are not universal to all musics (quite the reverse). The idea of studying performance expression is often a study of performer's action in dimensions that are un(der)-specified in scores. Understanding what constitutes performance expression can enhance performance pedagogy, suggest ways of humanising synthesised music better, but also:
"Use of expression can also tell us about the internal state of the performers themselves, reflecting their mental representations of the music, the motor plans they use to convert these mental representations into a sequence of well-timed movements" (Thompson Music, Thought, and Feeling pg. 239)
The work of Johan Sundberg about the human voice. Starting ~33 minutes he describes studies about performing music:
Visualising how different pianists perform the same musical phrase. (You can also check some of W. Goebl's more recent project).
CMPCP - Research Centre for Musical Performance as Creative Practice and particularly the shaping music in performance strand.
Daniel Leech-Wilkinson, The Changing Sound of Music: Approaches to Studying Recorded Musical Performance (London: CHARM, 2009)