I recall a while back that lots of users of AMD/ATI Radeon 5xxx cards had reported a somewhat major improvement in FSX performance when Catalyst 11.6 came out. Since then there have been a few newer Catalyst releases, the current one as of this post being 11.9, is anyone aware of any additional performance/quality improvements or differential with these releases?

The catalyst 11.9 is good. Also use Bojotoe's shader 3.0 model for the clouds.