You can configure NDMP a couple of ways and it's actually easy to silently get it wrong. Are you sure you are using an NDMP storage unit and not just an NDMP policy type? If you have an NDMP policy, but a Media Manager storage unit, the data flows via the LAN from the storage controller to a media server, and then to disk or tape. If you have an NDMP policy *and* an NDMP storage unit, the data flows from the storage controller directly to the disk or tape. We have some of each configured here depending on whether we've run fibre directly to our NDMP NetApps (I have noe experience with Celerras). On our NetApps, we have a command to show the tape stats and it shows the throughput - check to see if the Celerra has something similar.
For optimal performance (but more money), you would not use NDMP at all. You can use a Windows client to back up the data using a standard policy and store that in a de-duped storage unit. Then do daily incrementals forever and use synthetics to create your fulls. You'd never have to do a full again. Now this assumes you have the backend disk for your de-dupe pool, and that you have the software licenses in place. It also assumes you have great connectivity between that Windows client to the Celerra (10GigE would be ideal especially for that first full).