The 9i and 10g average backup time for ~600G/900G seems within range for a 4 cpu server using NFS.
Try Full backup + incremental level 0 using "block-tracking".
PS: Compression in 10g only increases the backup time.

Code:
ALTER DATABASE ENABLE BLOCK CHANGE TRACKING 
  USING FILE '/mydir/rman_change_track.f' REUSE;
For 11g the way to go is compressed backups, our 6-8 hour 10g backups were reduced to two hours after the migration to 11g for ~900G database.