I was using the Clarinet in A which is required in RK's Scheherazade. If you use Sibelius with GPO4 and just add the intro to movement 1, the clarinet is sounds totally out of place (i.e. off key). I know about transposing instruments, but none of the other instruments suffer from this when played back through Sibelius 6.1.
So my question is, is the Clarinet in A really a clarinet in A?
I've been a GPO user since February (GPO3) and upgraded to GPO4 in September. Neither app's instrument list shows an A Clarinet - only Bb and Eb. From what you write, and FWIW, it sounds as if Sibelius is using some sort of "intelligent pitchbend" (I don't have Sibelius but I've heard that it's supposed to be quite intelligent when it comes to playback).
One possible solution might be to create a copy of the Bb sfz file (don't edit the original file), open up in your favourite text editor and change the lokey variable of the first Region statement so that it's one semitone lower - which how Garritan appear to have derived the Player instruments (i.e. "pitch stretching"). Having said that, I wouldn't have the faintest idea how you would get Sibelius to recognize the new instrument (though, given my comment above, I suspect that it will do so automatically if the file is in the default folder).
The above is all imparted, of course, in the hope that one of the many fine experts on this Forum will wade in screaming if I've screwed up, and that nobody will want to claim against my (non-existent) Public Liability Insurance!