The idea that the composer best perform his piece with his own orchestra

1. Indeed. He knows his musicians best, and has written his piece with their powers in mind. In this “proper” (not: original) performance he can “correct” the orchestra, and, perhaps apply newly acquired insights to his score. As long as such proper performance is a viable possibility the claim in the title is of aesthetic relevance.

3. Within our listening culture one could argue, in line with Stephen Davies, that performances strive towards an ideal performance: towards good music, good circumstances, good instruments, and a matching with what one might suspect to have been the best way avaliable to contemporary listeners to hear and enjoy the music.
Or, one might argue, against the epistemological argument inherent in this approach, that the artistic norms applicable are, and must be ours. There is bound to be more than one successful performance of a piece. (Cf. Scruton in a BBC program on authentic performance; and Young).

Davies, Stephen. 2002. “Authenticity in Musical Performance.” In Arguing about art. Contemporary Philosophical Debates. Second edition, edited by Alex Neill and Aron Ridley, 57–68. London: Routledge.
Wollheim, Richard. 1993. “Pictorial Style: Two Views.” In The Mind and its Depths, 171–184. Cambridge (Mass.), London (England): Harvard University Press.
Young, James O. 2002. “The Concept of Authentic Performance.” In Arguing about art. Contemporary Philosophical Debates. Second edition, edited by Alex Neill and Aron Ridley, 69–80. London: Routledge.