I now have 14.5 TB of real useable space; using 4 Terabyte drives. I offloaded my 4 month old 3TB drives into a cheap 4 bay eSATA RAID box. That right there is another 9TB to play with under RAID5.
Just a quick test with Black Magic Speed Test.
Restoring from backup isn't so bad. I'm sustaining and averaging 200 MB/sec from various external STAE129 Thunderbolt drives. This restore consists of mostly small text,photos, and source code files via rsync. The restore is also taking a few hours, so peaking at 200 MB/sec for that long of a period is very good in my book. 3TB worth of data takes about 7 hours or so to restore. I remember that use to take 1-2 days.