In my company ( http://newzik.com ) we're hardly working on nicely displaying MusicXML files. And we like using Sibelius in our musician life.
But it seems that Sibelius MusicXML compliance is quiet limited/bugged, and as MusicXML support grows for mobile apps, it may be good for Sibelius to improve its compliance.
We would ever be ready to send you many technical bug reports with .sib and .xml files if we knew you really want to improve MusicXML compliance (at least export!).
Thanks in advance