Snapshots will give you faster recovery time
Offhost backup will remove impact on your production service/applications
VTL will make (virtual) tape backup much faster
And you can then export backups from VTL to ATL
... or, are you already doing that, when you said "we use ... in combination with tape backups ..."

Any reason you don't use VVR

Abe

Yes, RealTime has uses this large.  The advantage to RealTime (over CPS) is that it works on a SAN and block level.  So the initial sync goes directly from storage to RealTime without going through the host.  It literally creates a mirror volume of the exact data and then the changes to the blocks of that mirror on the protected storage are copied in to a journal on the RealTime server where they can be applied to the journal for any point in time recovery.  The advantage is that it doesn't have to resync so if you have a low data change rate it won't have to touch the original 20TB.  If the server goes down it will eventually perform a resync but not of the whole thing - just the changes.

Recovery can be done very quickly as well since data doesn't even need to be copied back from RealTime.  Instead the RealTime server will give a virtual disk volume to the host where the data can be accessed immediately (no copy or restore back unless needed). 

And for the point on VVR - RealTime will have the VVR (or block level asyncronous replication) built in to the next release but that would also be a good product to consider if you have issues syncing large data sets.