well i made a new zimbra server using 6.0.5 FOSS 32bit on centos 5.4
pc is running quad core 2.5ghz and 4gb ram and 1000gb es.2
configured with 100gb /, 100gb /opt/zimbra [lvm] and 100gb /backup
i have restored a mailbox (mine) using the zmmailbox postRestURL tgz method and the server seems really quick (compared to the 5.0.16 i was running on, dual core crappy HP box thing).
the new 6.0.5 seems really good