Haha, so we're on the same page RE: iSCSI exposure to other servers then.
RDM's for your repository VM's then? For our Windows repositories they're just vmdk's but it hasn't mattered whether that, CIFS shares, whatever. At the OS level we could pull data from any storage down to the tape server at speed. Backup to tape from tape server at speed. Pull files across the network to tape is slow. Anyways, sounds like a very similar setup to everyone else who's experiencing the same issue. Get you a support ticket going just to make sure nothing is missed, etc. They may do some registry hacks for Veeam to tune it a bit and investigate your tape server's NIC settings.
As a test, stick you a large file or two on your tape server, big as you can fit, and run a test file to tape job from there. See what your speed difference is.