After experimenting with the Xsample Chamber Ensemble library, I have to say that the integration of this library within Sibelius is superb! It is great having these very high quality sounds at your disposal while composing within Sib. And the sound quality really is that good (IMHO, of course). I've heard that they are working on brass libraries as well, so when they have released those Xsample, in my view, is the one library to have if you are looking for high quality solo instruments that are capable of emulating contemporary playing techniques.

Although the playback in Sib is great, I do want to tinker around with the mockup in Logic 8. And I just found out that it is possible to export a MIDI file of your composition that is optimized for the soundset you are using. So, when exporting for the Xsample soundset, all Keyswitches (or, in the case of Xsample, the cc0 control changes), modulation changes, etc., are saved in this midi file and can be read and modified within Logic, using hyperdraw. This saved me a lot of work, and enables me to just focus on those parts that I want to adjust.

So, thanks for this option, Sibelius, and a great thank you to Xsample (and no, I do not work for them, and neither am I in any way affiliated to them, just a very happy costumer)!