In oracle release notes for patch Oracle says that there could be a performance problem if SYS schema has statistics.They recommend deleting stats prior to upgrade and recreating them again after upgrade completed. There some ambiguous docs on Metalink regarding this issue. One states that one should not generate stats on sys schema unless there is a good reason for it. (wonder what that would be). Another states that unless DB has unusuall high number of objects in it SYS stats should be used. However they do not give you any specifics.
Is it another "TRY IT FOR YOURSELF " feature from Oracle? Did someone has done any research on it?