I just finished upgrading the OS on our Opteron 148 from Redhat9 to
Fedora FC2 X86_64 with full recompiles of Postgres/Apache/Perl/Samba/etc.
The verdict: a definite performance improvement. I tested just a few CPU
intensive queries and many of them are a good 30%-50% faster.
Transactional/batch jobs involving client machines (i.e. include fixed
client/networking/odbc overhead) seem to be about 10%-20% faster
although I will need run more data through the system to get a better
feel of the numbers.