We use the hardware de-dupe on our Quantum disk array, I have been told by Commvault's sales people on recent visits that it (Simpana) will provide similar results. Was have 200 TB of server backups being stored on ~19TB physical - the biggest bene of SImpana is you can de-dupe that tape data also - so you save on tape costs. The bene of the hardware de-dupe on the Quantum is that it offloads it to the Quantum, with Simpana you have a server (Media agent) doing the work. Your data will ultimately provide the reduction rate but overall I would say you should be able to get at least 5:1 reduction
John