My backup disk contains 2.4TB of 7z files. Each file is a website backup, a database backup or a 7z backup of an entire virtual machine / server. Many of these backups are of the same thing but at different points in time. For instance, my Windows 7 Virtual machine is backed up 7 times for a total of 961GB after 7zip compression.
Looking at my latest Windows 7 back up of Virtual Disks (My virtual C drive and data drive):
238GB Without Compression or dedupe
So looking at an individual backup, 7zip clearly wins - although it takes 24 hours to compress a 238GB backup on my 4 core 3.9Ghz 32GBs RAM machine (detailed here).
1.00x original files 1.04x zfs dedupe
However, if I put the complete 2.4TBs (After 7z compression) onto the ZFS filesystem and decompress it as it copies over to the ZFS partition I get 2.52x :)