I'll let Veeam reply to how tape interacts with the proxy as we are tape-less. I just wanted to chime in to ask if you have looked at Cisco's S series storage server? Sounds like it may fit your setup. This will probably be the route I go next year when my FAS comes up for refresh. ( we are however a all Cisco UCS shop) http://www.cisco.com/c/en/us/products/s ... index.html
I run a Netapp FAS connected over FC to my infrastructure for my primary backups. All my proxies etc are virtual. Backup copies are then copied over 10GB to my Netapp AltaVault that is synced to Azure storage for long term retention. I'm getting about a 14x dedupe/compression on the Altavault, just over 110TB uncompressed/deduped but only 8TB sitting on the appliance and in the cloud. That's another option to look at if you want to ditch tape, works pretty slick.