I'd think "definitive" makes most sense when it means the performance against which others have to be measured. It doesn't rule out the possibility that a performance is going to come about that excels it. (In which case does that one become "definitive", and the other once isn't any more?)

And I think it should always be understood as "definitive for me" because it's such a subjective thing. (For example in that last post I gave two links for Fairy Tale of New York, and for me Christy Moore's would be the definitive one - but I know for many people it'd have to be the other one with the Pogues and Kirsty McColl.)