First off, you need to understand that NDMP performance has nothing to do with BackupExec in 90+% of cases. NDMP is a set of commands executed remotely by the backup server, telling the NDMP device what to do.
In the case of a NetApp filer, BE tells the FAS2020 to backup a certain LUN, in which the controller head does a UFS_DUMP, and streams that dump over the network or via FC to your target. Thus, in many cases, you are limited by the speed of your disk, bandwidth, and the CPU power of the filer itself. BackupExec only receives the stream as fast as your filer can create and spit it out.
There are commands to verify the throughput of the filer that you can run via CLI within the ONTAP OS. I dont recall them off-hand, but if I remember later, I have it in some notes somewhere, I'll post an update. This gives you a good baseline for what your device *could* theoretically put out via NDMP.
FYI, the FAS2020, is one of the lower powered models, with I believe a single Intel Mobile Celeron processor and just a single GB of memory in its base config (my 2yr old laptop is more powerful than your filer). The 2050 too is equally a lower model, even though it has a higher model #. The best in the FAS2000 lineup is the FAS2040, with a Xeon processor, and more RAM.
If you need to test your tape throughput, the baseline you can do is to copy a few hundred GB's of mixed data (both large files, and little ones) to your local HDD's on your media server, and back that up, to get a better idea of the throughput of your tape.
There was also a posting in the BLOG section I believe in the forums for tuning LTO, that would be handy for you to look at as well to verify your tape settings are the most optimum for your particular setup. https://www-secure.symantec.com/connect/articles/tuning-my-lto4-tape-drive
Once you've identified how fast your tape can write data, and then figured out how fast the FAS2020 can read and dump data; more than likely you may need to play with aggregates within the filer to get more performance.
Note, there are a number of posts on the NetApp forums about FAS2020 performance issues. Best to check some of those as well, to see what was done to reduce the CPU load on this particular filer.
