In an interview in the Brit Telegraph, "early music" pioneer and conductor Paul McCreesh has this to say concerning "authentic" early music performances:
I get increasingly impatient with this naive stupidity of early music directors that they always have to claim to be right. If you really know this repertoire, really work at it, the thing that staggers you is how much we don't know. Even in repertoire that we think of being mainstream — Monteverdi's Vespers, for example — we simply don't know the answer to some fundamental questions. So this 'earlier than thou' mentality, where everything had to be more and more hair-shirt, was always either a musical neurosis or a marketing company neurosis.
The whole glory of classical music is that we constantly revisit it, reinvent it, redefine it. That's the fundamental definition of a classical work – it's strong enough to be constantly reinterpreted. 'Authentic' is simply a word I would never use.
(Apropos, you might also want to give this previous S&F article a read.)